The Embryonic Tail, Lanugo, and the Pronephros

The human embryo builds a segmented tail and then kills about five pairs of its segments, the strongest sign of ancestry here. Lanugo's one proposed job, holding vernix, has never been tested. A human pronephros probably never forms; the mesonephros does, and its duct is needed for the permanent kidney and male ducts.

By AI (Claude Opus 5.5, reasoning effort 40) Draft:  September 2026

"The higher Vertebrates pass through an embryonic stage, in which they possess first a pronephros and then a mesonephros, which is an irrefragable proof that in their ancestors, and consequently in the ancestors of Man, each of these organs once constituted in turn a permanent urinary system." — Robert Wiedersheim, The Structure of Man, 18951

Summary

Three structures get three answers under the test set in Embryonic Leftovers:  what the structure does, and whether its route is explained by function, by ancestry, or both.

  1. The tail is real, and its removal is the best evidence here for ancestry. From about day 32 to day 42 after fertilization the embryo grows a free tail behind the cloaca, made of "paired somites, mesenchyme and extensions of the neural tube, notochord and gut."2 The most careful count found 13 to 16 pairs of somites in the tail region at about day 40, and about five pairs fewer a few days later.3 Cell death removes the free tail by the eighth week.2 The caudal eminence that builds it has real work, the lower spinal cord and the end of the notochord. Nothing known requires making segments and then killing them.
  2. Lanugo is a fetal coat whose function is asserted, not measured. It spreads between about weeks 12 and 26 and is mostly shed before birth. Textbooks say it holds vernix, the waxy coating of the fetal skin; the only evidence found is that vernix gathers where lanugo is dense. The claim that fetal monkeys grow the same coat has no source either. Neither side has the data.
  3. The human pronephros probably does not exist. A 2019 study of 43 embryos found none at any stage and traced the textbook one to projecting fish and frog kidneys onto human embryos.4 So "the human pronephros does nothing" is a claim about an absent structure. The mesonephros does exist. It makes urine from about six weeks, and its duct gives rise to the bud that induces the permanent kidney and later becomes the male sperm ducts. Its route is explained by function.

Admitting a useless part costs the design argument one thing here:  the dying tail segments have no known job while they exist.  A design account must find that job or accept that the human program builds and trims a segmented structure found, in full, in tailed animals, which is what common descent predicts.  Two stock claims also fail:  the regressing tail has no "bones" to reabsorb, as Jerry Coyne says it does, and "roughly forty published cases" of tails at birth matches no count found.

Part 1: The caudal eminence and the embryo's tail

What the caudal eminence builds

The caudal eminence is the remnant of the primitive streak at the hind end of the embryo.  Keith Moore's textbook notes that at the end of the fourth week "A long tail-like caudal eminence is also a characteristic feature."5  Fabiola Müller and Ronan O'Rahilly, who reconstructed 52 staged embryos, found it "most active developmentally during stage 13, when most of the required (ca 6 out of 9) pairs of somites appear."6  It builds the rest of the body's hind end by forming "structures (notochord, postcloacal gut, neural cord/neural tube) from mesenchyme," a process called secondary neurulation that runs from stage 12 to stage 17 or 18.6  H. J. Yang and colleagues found that the caudal neural tube reached "the tip of the caudal portion of the embryo" by stage 17, that the gut behind the cloaca had disappeared by stage 18, and that the neural tube in the tail showed "involution at stage 19."7

So the eminence is not idle.  It makes the lower spinal cord, the end of the notochord, and the somites that become the sacrum and coccyx.  Some of what it makes is then removed.

What the embryo builds and then removes

The 23 Carnegie stages cover "the first 8 postfertilizational weeks."8  Moore's figure captions give stage 14 as about 32 days, stage 16 as about 40, stage 17 as about 42 and stage 21 as about 52.5

J. F. Fallon and B. K. Simandl studied the tail from stage 14 to stage 22.  At stage 14 it "is composed of paired somites, mesenchyme and extensions of the neural tube, notochord and gut."  It "grows with the embryo through early stage 17 when it extends more than a millimeter from the trunk."2  Then "during stage 17 cells begin to die in all structures throughout the tail," reaching massive numbers at stages 18 and 19, and "By late stage 21 or early stage 22 there is no free tail."2  Part of the shortening is the trunk growing over the tail's base, and they say so.

How many segments are lost has long been disputed.  Jan Langman's textbook gave "42 to 44 pairs" of somites, including "8 to 10 coccygeal pairs," and said "the last 5 to 7 coccygeal somites later disappear."9  O'Rahilly and Müller rejected that:  "The full complement comprises about 38 or 39 (rather than 42-44) pairs," with "usually 4 or 5 coccygeal."10

Sayaka Tojima's team at Kyoto settled much of this in 2018 by reconstructing every somite in three dimensions in 42 embryos.  Total somite pairs rose from 30 at stage 13 to a peak of 39 to 41 at stage 16 and were "then reduced by four or five between CS16 and CS17."  In the region behind the hind limb buds, pairs peaked at 13 to 16 at stage 16, then fell by "about five pairs," after which 7 to 11 remained.3  They tie the loss to cell death, citing Fallon and Simandl and their own unpublished stains.  The samples are small, six embryos each at stages 16 and 17.  It is the only three-dimensional count, and it roughly matches Langman's five to seven segments lost.

Is "tail" the right word?

Müller and O'Rahilly say the caudal eminence "does not produce even a temporary 'tail' in the human."6  Their textbook explains why.  A tail, "in ordinary usage and with reference to vertebrates, consists of 'a number of gradually attenuated coccygeal vertebrae covered with flesh and integument' (OED).  The human embryo possesses no (vertebrated) 'tail' at any stage of development."10  By that definition they are right.  No vertebra in the tail is bone when the tail regresses.  Bone begins to form in the arches of the neck vertebrae "about the seventh or eighth week," and the first coccygeal segment ossifies "between the first and fourth years" after birth.11  That also corrects Jerry Coyne, who wrote that the embryonic tail disappears as "its bones and tissues are simply reabsorbed."12  There are no bones to reabsorb.

Tojima's team used a different definition:  "we defined 'tail' as an elongated trunk posterior to the cloaca, as in other amniotes."3  By that definition the human embryo has a tail.  The dispute is not only about words.  O'Rahilly and Müller also made a factual claim:  "The coccygeal somites at first fail to reach the end of the body," and the tapered tip "contains merely neural tube."10  Tojima's reconstruction found the opposite:  "caudal somites were distributed throughout the tail to the tip."3  On the newer and more complete data, the embryo builds a segmented extension behind the cloaca and removes the end of it.

Tailed mammals and apes

Tojima's team found that the human embryonic tail "is formed just as in other tailed amniotes," and that a similar loss of caudal somites occurs in other short-tailed amniotes.3  In tailed mammals the same tissue keeps making somites, and genes tune the count:  in mice, "Hoxb13‐knockout mice had longer tails and two additional caudal vertebrae."3  No study of tail regression in ape embryos was found.  The Coccyx covers the adult coccyx, the muscles it anchors, and a 2024 study of a genetic insertion shared by apes and humans that shortens mouse tails.

Tails at birth

Atavisms covers babies born with tail-like appendages:  "true" tails have muscle but no vertebrae, and bony ones carry a normal coccygeal count.  Two points belong here.  First, the count.  Anh Dao and Martin Netsky reviewed "33 previously reported cases of true tails and pseudotails" in 1984.13  Frank Lu and colleagues found that "In reviewing the literature from 1960 to 1997, 59 cases were described."14  "Roughly forty published cases" matches neither count, and the literature has grown since.  Second, Tojima's finding that segments reach the tail tip undercuts the old idea that boneless tails at birth are a retained boneless embryonic tip.  The team concluded that "it is still unclear whether the 'true tail' is derived from the embryonic tail."3

Part 2: Lanugo

When it grows and goes

Lanugo is fine, soft, lightly pigmented fetal hair.  Moore: "Lanugo begins to appear toward the end of the 12th week and is plentiful by 17 to 20 weeks."5  Z. Domagała's team in Wrocław examined 278 fetuses and found that on the trunk "the first single hairs occur on the posterior wall of the trunk in the 17th week of fetal life," with the pattern completed on the back by week 23 and on the abdomen by week 26.15  Susan Blackburn's physiology text says "Lanugo is shed before or after delivery and replaced by shorter, coarser hairs (vellus hairs) from new follicles."16  Hair shed in the womb is swallowed:  "Meconium consists of vernix caseosa, lanugo, squamous epithelial cells, occult blood," and more.16  Darwin noticed in 1871 that the coat covers the whole body except the palms and soles, "like the inferior surfaces of all four extremities in most of the lower animals," and concluded that it was "the rudimental representative of the first permanent coat of hair in those mammals which are born hairy."17  His argument was the pattern of the coat, not its uselessness.

What it is said to do

Holding vernix.  Vernix caseosa is the greasy film of skin-gland oil and shed cells that coats the fetus.  Moore says it "protects the delicate fetal skin from abrasions, chapping, and hardening that result from exposure to the amniotic fluid," and that lanugo "helps the vernix to adhere to the skin."5  Philip Brauer's review book says "the lanugo hair helps retain it on the outer skin surface."18  The evidence offered is one observation:  "The vernix has a tendency to accumulate at sites of dense lanugo growth."16  The timing fits, since vernix forms "after 17 to 20 weeks" and "After 36 weeks, the vernix thins and begins to disappear," the same span in which lanugo spreads and is shed.16  But a correlation is not a test.  No study was found that compares vernix in fetuses with and without lanugo.

Warmth.  No source found claims lanugo warms a fetus.  Coyne's argument that the womb is "a cozy 98.6 degrees Fahrenheit"12 answers a claim nobody makes.

Sensing and growth.  K. Bystrova proposed in 2009 that "Repeated oscillations of lanugo hairs encased by vernix caseosa during fetal movements in amniotic fluid activate highly sensitive mechanoreceptors" that speed fetal growth.19  It appeared in Medical Hypotheses and reports no data.

Protecting the skin is the vernix's job;  lanugo's part in it is plausible and untested.

When lanugo stays

In congenital hypertrichosis lanuginosa, "the lanugo distribution continues to progress, involving the whole body," and the disorder has been reported with abnormalities of chromosome 8q.20  Robert Wiedersheim in 1895 called such persistence "a return to a primitive hairy condition in Man."1  Piero Pavone's 2015 review reports the modern version, "an atavistic reversion of a suppressed ancestral gene," and adds for the idiopathic form that "To date, a clear specific molecular abnormity has not been proved."20  Adults with anorexia nervosa grow something else.  R. Strumia describes "Lanugo-like body hair" that is "pigmented," "not a sign of virilization," and "associated with decreased activity of the 5-α-reductase enzyme system, probably due to hypothyroidism."21  That is a starvation response in adult follicles, not fetal lanugo returning.

Other primates

Coyne's case needs the comparison:  "fetal monkeys also develop a coat of hair at about the same stage of development.  Their hair, however, doesn't fall out, but hangs on to become the adult coat.  And, like humans, fetal whales also have lanugo."12  He gives no source.  The Center for Academic Research and Training in Anthropogeny at UC San Diego, a secular database of human-ape differences, notes that "great apes are born with a considerable amount of regular hair" and asks, "Do great apes have lanugo hair in an earlier stage of embryogenesis?"  It lists no publications on the question.22  It also notes that lanugo is found in "some other new-born mammals such as seals and elephants."22  The comparison that would decide between "primate remnant" and "skin tool" has not been made.

Part 3: Three kidneys or one

The claim

Wiedersheim called the pronephros-then-mesonephros sequence "an irrefragable proof" of ancestral kidneys.1  Coyne writes that "the human embryo actually forms three different types of kidneys, one after the other, with the first two discarded before our final kidney appears.  And those transitory embryonic kidneys are similar to those we find in species that evolved before us in the fossil record— jawless fish and reptiles, respectively."12  In 2024 he listed "the developmental sequence of our kidney" and "our transitory coat of hair in utero" among traits with no adaptive explanation.23  Moore's textbook still describes "Three sets of successive kidneys."5

Is there a human pronephros?

Probably not.  B. S. de Bakker and colleagues examined serial sections of 43 human embryos from stage 8 to stage 23, with extra embryos at the stages when a pronephros is said to appear.  A true pronephros has nephrons whose glomeruli sit outside the tubules and filter into the body cavity.  They found none:  "a true pronephros is not detectable in human embryos although the most cranial part of the amniote excretory organ is often confusingly referred to as pronephros."4  They trace the textbook structure to a 1912 study by Felix and to naming habits from "an era in which study designs were based on the theory that ontogeny recapitulates phylogeny."4  They conclude that "The elements for a functional pronephros ... are undeniably never present."4  O'Rahilly and Müller reached the same view earlier:  at the level of the head-end somites "tubules do not form, and 'the concept of the pronephros . . . does not apply to the human embryo' (T.W. Torrey)."10

Alfred Romer and T. S. Parsons's comparative anatomy text agrees.  "Pronephric units in mammals are at best simple structures, without glomeruli and often rudimentary."  In amniotes the pronephros "never survives in the adult, and, except for its association with formation of the duct, it hardly deserves a separate name."24  What remains is the duct.  It forms first at the head end, grows back to the cloaca, and "most parts of the ducts persist and are used by the second set of kidneys."5

The mesonephros works

O'Rahilly and Müller treat the kidney as one field:  "The kidney is best regarded as a unit (the holonephros), although temporally and spatially two developmental components are involved: the mesonephros and the metanephros ... These constituents act, respectively, as temporary and permanent excretory organs."  They add that "The mesonephros begins to produce urine at 6 weeks."10  Moore says it functions "as interim kidneys for approximately 4 weeks."5

Its duct matters at least as much as its filtering.  "The ureteric bud is a diverticulum (outgrowth) from the mesonephric duct near its entrance into the cloaca."  The bud branches into the collecting tubules, and "The end of each arched collecting tubule induces clusters of mesenchymal cells in the metanephrogenic blastema to form small metanephric vesicles," which become the nephrons of the permanent kidney.5  Romer and Parsons put it plainly:  "the ureter precedes and even induces the tubules embryologically."24  In males the duct then becomes "the duct of the epididymis and the ductus deferens," and some mesonephric tubules become the efferent ductules of the testis.10  The permanent kidney begins in the fifth week and makes urine about four weeks later.5  So the order is forced by the task.  The duct must exist before the bud that grows from it, and the bud must exist before the kidney it induces.

One caution:  de Bakker's team notes that with a yolk sac or placenta, "kidney development is not essential for waste disposal or osmotic regulation prior to birth."4  Whether mesonephric urine does anything essential is not settled by the sources found.

Coyne's comparison, corrected

The mesonephros is not a reptile kidney.  Romer and Parsons write that "it is perhaps best to confine the term mesonephros solely to the kidney of embryonic amniotes.  The adult anamniote kidney may be termed the opisthonephros."24  Reptiles are amniotes, and their figure labels the adult kidney of that group "Metanephros of amniotes."24  A pronephros does serve as the adult kidney "In basal chordates, e.g., Amphioxus, Cyclostomes, and Dipnoi," and as the embryonic kidney in bony fish and amphibians.4  But a human has none.  Coyne's broad point, that fish and frog larvae use kidney types amniotes use only as embryos or not at all, is correct;  his pairing is not.

What is left for ancestry

Romer and Parsons wrote that the differences between the three "are readily explainable on functional grounds; the three appear to be regionally specialized parts of the original holonephros, which serve different functions."24  They were evolutionists, and "original" means ancestral.  De Bakker's team, also evolutionists, read the missing human pronephros as loss after the yolk sac and placenta appeared.  Neither reading is tested by human embryology.  A single nephric band built front to back, duct first, is also what a designer solving this task would build, so the kidney sequence does not discriminate between the models.

Part 4: The argument and the replies in their own words

The argument.  Darwin argued from the pattern of the fetal coat, Wiedersheim from the kidneys, and Coyne from all three.  Coyne adds a mechanism:  "the descendant inherits the developmental program of its ancestor ... Many structures that form later in development require biochemical 'cues' from features that appear earlier."12  That second sentence concedes the kidney point made above:  an early structure can be kept because a later one needs it.

Answers in Genesis.  David Menton's principle is that "There are several structures that function during the development of the embryo and fetus that appear to be no longer used after birth ... Such structures perfectly fit the definition of a vestige, but they are not vestiges of evolution."25  That fits the mesonephros.  It does not fit the tail segments, which die before anything could use them.

Jay Wile on lanugo.  Wile, a young-earth creationist writer, was right that Coyne treated warmth as the only possible use:  "He seems to think that the only reason a human embryo would need hair is for insulation."26  He then went beyond every source he cites:  "Without lanugo, the embryonic skin would not form the protective layer that it needs while in the womb."26  Nothing he cites, and nothing found here, tests that.

Wile and CMI on the tail.  Wile wrote that the structure "has nothing to do with a tail" and that the embryonic tail "has been thoroughly debunked in the scientific literature."27  Andrew Lamb of Creation Ministries International called it "the false idea that in the womb people have an 'embryonic tail'."28  Both rest on Müller and O'Rahilly.  Neither addresses Fallon and Simandl's 1978 dying tail, which predates both.  Lamb makes one fair point:  since the extra somites never develop, "it is pure evolution-inspired supposition to presume to know what they would develop into, and to label them 'coccygeal'."28  Tojima's team avoids that label and counts "caudal" somites.

Van Niekerk on the kidneys.  In CMI's Journal of Creation, E. van Niekerk quotes Romer and Parsons accurately apart from dropping "original" and adding a gloss.29  He concludes that "the simpler system has nothing to do with evolutionary history, and everything to do with the requirements of the embryo at the early stage," and says mammal embryos "briefly" show the pronephros setup.29  The conclusion holds for the mesonephros and its duct.  But the early-urine sentence he relies on sits on a page that says mammalian pronephric units lack glomeruli.24  His case would be stronger with de Bakker's finding that there is no human pronephros at all.

Objections and responses

"The leading human embryologists say there is no tail."  Müller and O'Rahilly define a tail by vertebrae, and by that definition they are right.  They also said segments do not reach the tail tip, and the only three-dimensional count found that they do.3

"The lost somites are relabelled or merged, not destroyed."  Tojima found the loss only in the tail region, and Fallon and Simandl saw cells dying "in all structures throughout the tail."2  The link between the count and cell death rests partly on unpublished stains, a real weakness.

"The caudal eminence has a function, so the tail has one."  The eminence's work explains why the eminence exists.  It does not explain why about five pairs of segments form and then die.

"Textbooks prove lanugo holds vernix."  Textbooks assert it;  the evidence found is a correlation.  It may be true and should be tested before either side leans on it.

"A designer could build a tail and remove it."  Logically, yes.  But a design model does not predict that a human embryo would build a segmented tail like a mouse's and then kill its end, and common descent does.

Issues and unanswered questions

  • The tail count rests on one team. Tojima's 42 embryos include one each at stages 13 and 15, and the cell-death link uses unpublished data. An independent three-dimensional count with cell-death markers would settle it.
  • No design account of the dying segments exists. No creationist source found engages Fallon and Simandl or Tojima. A design answer needs a job for those somites while they live, for example as a source of cells or signals for the sacral region, which lineage tracing in mouse or chick could test.
  • No ape embryo data. No study found compares tail regression or the fetal coat in ape embryos. Coyne's monkey and whale claims about lanugo are unsourced.
  • Lanugo's function is untested both ways. Fetuses that lack hair follicles could show whether vernix still forms normally; no such study was found.
  • The mesonephros's urine. O'Rahilly and Müller date it from six weeks, but de Bakker's team says a placenta makes fetal kidney function nonessential. What that urine does is not answered by the sources found.
  • The pronephros result is one group's histology, with two specimens per stage after stage 11. It agrees with O'Rahilly and Müller, but current textbooks still describe a human pronephros.

Verdict

Tail:  real under a stated definition, built by a caudal eminence that does necessary work, and trimmed by the death of about five pairs of caudal segments.  The work explains the eminence;  only ancestry, so far, explains the segments built and destroyed.  This is the one part of the three that does nothing known while it exists, and the design argument should admit it as an open problem rather than deny the tail.

Lanugo:  a real fetal coat, laid down between weeks 12 and 26 and mostly shed and swallowed before birth.  Its textbook job of holding vernix is plausible and untested, and the primate comparison that would support ancestry is unmeasured.  Neither side can claim it yet.

Kidneys:  the strongest case for function and the weakest for ancestry.  A human pronephros probably never forms, so it cannot be a useless leftover.  The mesonephros filters, and its duct is needed for the ureteric bud, the permanent kidney and the male ducts.  Common descent still reads the single nephric band as inherited, and embryology cannot test that reading.

Sources

  1. 1.

    Wiedersheim, Robert.  The Structure of Man:  An Index to His Past History.  Macmillan.  Translated by H. Bernard and M. Bernard.  1895.  Public domain.  Archive.org OCR in the source library;  pages from the running heads.

    1. 1a.

      Pages 187–188:  “The higher Vertebrates pass through an embryonic stage, in which they possess first a pronephros and then a mesonephros, which is an irrefragable proof that in their ancestors, and consequently in the ancestors of Man, each of these organs once constituted in turn a permanent urinary system.”  The sentence crosses the page break after and con-.  It is Wiedersheim's text, not one of the translators' bracketed additions. ↩ ↩

    2. 1b.

      Page 10:  “In all the cases mentioned above, the persistence of the vestigial lanugo must undoubtedly be regarded as a return to a primitive hairy condition in Man”. ↩

  2. 2.

    Fallon, J. F., and B. K. Simandl.  “Evidence of a role for cell death in the disappearance of the embryonic human tail.”  American Journal of Anatomy 152(1):111–129.  1978.  Copyright.  Abstract only (PubMed 677043, via Europe PMC), in the source library.

    1. 2a.

      Section Abstract:  “The tail is a prominent feature of the human embryo during stage 14 and is composed of paired somites, mesenchyme and extensions of the neural tube, notochord and gut.  The tail grows with the embryo through early stage 17 when it extends more than a millimeter from the trunk.” ↩ ↩

    2. 2b.

      Section Abstract:  “However, during stage 17 cells begin to die in all structures throughout the tail.  Cell death continues in the succeeding stages reaching massive numbers by stages 18 and 19, and the tail becomes less and less prominent with developmental time.”  “By late stage 21 or early stage 22 there is no free tail.  We conclude that cell death has a major role in the destruction of tail structures and the concurrent loss of the human tail.” ↩ ↩ ↩

  3. 3.

    Tojima, Sayaka et al.  “Tail reduction process during human embryonic development.”  Journal of Anatomy 232(5):806–811.  2018.  Copyright, Anatomical Society.  42 serially sectioned embryos of the Kyoto Collection, stages 13 to 23.  PMC copy in the source library;  locators are section names.  Mirror:  PMC

    1. 3a.

      Section Abstract:  “Although the human tail is completely absent at birth, the embryonic tail is formed just as in other tailed amniotes.”  “Tail reduction with a decrease in somites has also been observed in other short-tailed amniotes”. ↩

    2. 3b.

      Section Results:  “The total number of somite pairs increased linearly from CS13, at which time there were 30 pairs of somites, and peaked at CS16 (39–41 pairs).  The number of pairs was then reduced by four or five between CS16 and CS17.”  “there was a gradual increase from CS13, with a peak at CS16 of 13–16 pairs of caudal somites.  However, there was then a dramatic decrease between CS16 and CS17 of about five pairs of caudal somites, after which the number of caudal somite pairs did not change much, remaining at 7–11 pairs in the caudal region.”  Sample sizes from Table 1:  stage 13, 1;  stage 15, 1;  stages 16 and 17, 6 each. ↩ ↩

    3. 3c.

      Section Methods, Definition of the 'tail' in this study:  “In this study, we defined ‘tail’ as an elongated trunk posterior to the cloaca, as in other amniotes.” ↩

    4. 3d.

      Section Discussion:  “However, we found that caudal somites were distributed throughout the tail to the tip during development; thus, we showed that the ‘caudal filament’ without bony elements (somites) was not actually present.” ↩ ↩

    5. 3e.

      Section Discussion:  “Unfortunately, most previous reports have lacked such information; thus, it is still unclear whether the ‘true tail’ is derived from the embryonic tail.” ↩

    6. 3f.

      Section Discussion:  “Hoxb13‐knockout mice had longer tails and two additional caudal vertebrae”. ↩

  4. 4.

    de Bakker, B. S. et al.  “The Pronephros; a Fresh Perspective.”  Integrative and Comparative Biology 59(1):29–47.  2019.  CC BY-NC 4.0.  In the source library.  Serial sections of 43 Carnegie Collection embryos, stages 8 to 23.  Mirror:  PMC

    1. 4a.

      Section Abstract:  “Current knowledge of the human pronephros, however, appears to be based on only a hand full of human specimens.”  “Non-integrated nephrons were not identified in histological sections of human embryos.  Therefore, we conclude that a true pronephros is not detectable in human embryos although the most cranial part of the amniote excretory organ is often confusingly referred to as pronephros.” ↩ ↩

    2. 4b.

      Section Pronephros in human embryos?:  “In an era in which study designs were based on the theory that ontogeny recapitulates phylogeny (Smith 1953; Huettner 1968; Hiruma and Nakamura 2003; Solhaug et al. 2004), it could be condoned that the findings of studies on fish and amphibians were projected onto the early stages of human development.”  “Therefore, recent literature is almost always directly or indirectly referring to the extensive study of the human pronephros by Felix in 1912 (Felix 1912).” ↩

    3. 4c.

      Section Discussion:  “The elements for a functional pronephros, including a fully developed external glomerulus, hollow ciliated nephrostomes and hollow pronephric tubules, are undeniably never present.” ↩

    4. 4d.

      Section Evolutionary aspects of kidney development:  “Thus, in the presence of a yolk sac or a placenta as efficient waste disposal systems, kidney development is not essential for waste disposal or osmotic regulation prior to birth (Vize et al. 2003).” ↩

    5. 4e.

      Section Pronephros in anamniotes:  “In basal chordates, e.g., Amphioxus, Cyclostomes, and Dipnoi, the pronephros functions generally as the adult kidney (Prentiss and Arey 1917; Hamilton et al. 1972; Vize et al. 2003; Bertrand and Escriva 2011).  In bony fish and amphibians, the pronephros functions as the embryonic kidney.” ↩

  5. 5.

    Moore, Keith L. et al.  The Developing Human: Clinically Oriented Embryology.  Elsevier, Philadelphia.  10th edition.  2016.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 22, anchored by the index entries Pronephroi, 243 (leaf 265) and Lanugo, 95-97, 445 (leaves 118-119 and 467).  Ages are counted from fertilization.

    1. 5a.

      Page 75:  “A long tail-like caudal eminence is also a characteristic feature”. ↩

    2. 5b.

      Page 82:  “Carnegie stage 14, approximately 32 days”.  Page 386:  “Carnegie stage 16 at approximately 40 days”.  Page 83:  “Carnegie stage 17, approximately 42 days”.  Page 84:  “Carnegie stage 21, approximately 52 days”.  Figure captions 5-13, 17-7, 5-14 and 5-16. ↩

    3. 5c.

      Page 445:  “Lanugo begins to appear toward the end of the 12th week and is plentiful by 17 to 20 weeks”. ↩

    4. 5d.

      Page 96:  “The vernix protects the delicate fetal skin from abrasions, chapping, and hardening that result from exposure to the amniotic fluid.  Fetuses are covered with fine, downy hair, lanugo, which helps the vernix to adhere to the skin.” ↩

    5. 5e.

      Page 243:  “Three sets of successive kidneys develop in the embryos.  The first set, the pronephroi, is rudimentary.”  “The pronephroi soon degenerate; however, most parts of the ducts persist and are used by the second set of kidneys.”  “The mesonephroi function as interim kidneys for approximately 4 weeks, until the permanent kidneys develop and function”.  The OCR splits proneph- roi across a line;  joined here. ↩ ↩ ↩

    6. 5f.

      Page 245:  “Metanephroi, or the primordia of the permanent kidneys, begin to develop in the fifth week (Fig. 12-6) and become functional approximately 4 weeks later.”  “The ureteric bud is a diverticulum (outgrowth) from the mesonephric duct near its entrance into the cloaca”.  “The end of each arched collecting tubule induces clusters of mesenchymal cells in the metanephrogenic blastema to form small metanephric vesicles”. ↩ ↩

  6. 6.

    Müller, F., and R. O'Rahilly.  “The primitive streak, the caudal eminence and related structures in staged human embryos.”  Cells Tissues Organs 177(1):2–20.  2004.  Copyright.  Abstract only (PubMed 15237191), in the source library.

    1. 6a.

      Section Abstract:  “The caudal eminence is most active developmentally during stage 13, when most of the required (ca 6 out of 9) pairs of somites appear.” ↩

    2. 6b.

      Section Abstract:  “Primary neurulation lasts from stage 8 to stage 12, secondary from stage 12 to stages 17 or 18. (11) Secondary development and secondary neurulation are characterized morphologically by direct formation of structures (notochord, postcloacal gut, neural cord/neural tube) from mesenchyme.” ↩

    3. 6c.

      Section Abstract:  “The eminence produces the caudal part of the notochord and, after closure of the caudal neuropore, all caudal structures, but it does not produce even a temporary 'tail' in the human.” ↩

  7. 7.

    Yang, H. J. et al.  “Secondary neurulation of human embryos: morphological changes and the expression of neuronal antigens.”  Child's Nervous System 30(1):73–82.  2014.  Copyright.  Abstract only (PubMed 23760472), in the source library.  21 embryos, stages 12 to 23.

    1. 7a.

      Section Abstract:  “The formation of the caudal neural tube to the tip of the caudal portion of the embryo was finished at stage 17.  The postcloacal gut had completely disappeared at stage 18, and multiple cavities of the caudal neural tube were clearly visible.  The caudal portion of the neural tube showed findings suggestive of involution at stage 19.” ↩

  8. 8.

    O'Rahilly, Ronan, and Fabiola Müller.  “Developmental stages in human embryos: revised and new measurements.”  Cells Tissues Organs 192(2):73–84.  2010.  Copyright.  Abstract only (PubMed 20185898), in the source library.

    1. 8a.

      Section Abstract:  “Twenty-three stages cover the embryonic period, i. e. the first 8 postfertilizational weeks of development.” ↩

  9. 9.

    Langman, Jan.  Medical Embryology.  Williams & Wilkins, Baltimore.  4th edition.  1981.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf 78 is page 60 on the item's BookReader page map, which also puts the index entry Odontoblast, 291 on leaf 311;  the index lists Somites, 59, 60.

    1. 9a.

      Page 60:  “From here new somites appear in craniocaudal sequence, approximately three per day, until at the end of the fifth week 42 to 44 pairs are present (figs. 5-4 and 5-6).  These are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs.  The first occipital and the last 5 to 7 coccygeal somites later disappear.” ↩

  10. 10.

    O'Rahilly, Ronan, and Fabiola Müller.  Human Embryology & Teratology.  Wiley-Liss, New York.  3rd edition.  2001.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 16, anchored by the index entries Talipes, 120, 124, 128, 391 (leaves 136, 140, 144, 407) and TAR syndrome, 390 (leaf 406).  Reference superscripts are lost in the OCR.

    1. 10a.

      Page 360:  “The full complement comprises about 38 or 39 (rather than 42-44) pairs but they are never all visible at one time: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and usually 4 or 5 coccygeal.” ↩

    2. 10b.

      Page 360:  “The coccygeal somites at first fail to reach the end of the body.  The caudal tip of the trunk appears particularly tapered at 5-6 weeks (Fig. 18-7), because it contains merely neural tube.” ↩

    3. 10c.

      Page 361:  “A tail, in ordinary usage and with reference to vertebrates, consists of "a number of gradually attenuated coccygeal vertebrae covered with flesh and integument" (OED).  The human embryo possesses no (vertebrated) "tail" at any stage of development.” ↩

    4. 10d.

      Page 299:  “At the level of the rostral somites, although the intermediate mesoderm becomes hollowed (as nephrotomes and nephrocoels), tubules do not form, and "the concept of the pronephros . . . does not apply to the human embryo " (T.W. Torrey).” ↩

    5. 10e.

      Page 299:  “The kidney is best regarded as a unit (the holonephros), although temporally and spatially two developmental components are involved: the mesonephros and the metanephros (Fig. 15-1).  These constituents act, respectively, as temporary and permanent excretory organs.”  “The mesonephros begins to produce urine at 6 weeks.”  “The mesonephric duct becomes the duct of the epididymis and the ductus deferens.” ↩ ↩

  11. 11.

    Gray, Henry.  Anatomy of the Human Body.  Lea & Febiger.  20th edition.  Edited by Warren H. Lewis.  1918.  Public domain.  Archive.org OCR in the source library;  pages from the running heads.

    1. 11a.

      Page 111:  “Ossification of the vertebral arches begins in the upper cervical vertebrae about the seventh or eighth week of fetal life, and gradually extends down the column.”  Page 114:  “The coccyx is ossified from four centers, one for each segment.  The ossific nuclei make their appearance in the following order: in the first segment between the first and fourth years”.  The OCR reads fetal hfe. ↩

  12. 12.

    Coyne, Jerry A.  Why Evolution Is True.  Viking, New York.  2009.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 26 from the item's BookReader page map, confirmed here by the index entry lanugo, 80 against the lanugo paragraph on leaf 106.

    1. 12a.

      Page 65:  “As we'll learn shortly, early in development human embryos have a sizable fishlike tail, which begins to disappear about seven weeks into development (its bones and tissues are simply reabsorbed by the body).” ↩

    2. 12b.

      Page 77:  “During development, the human embryo actually forms three different types of kidneys, one after the other, with the first two discarded before our final kidney appears.  And those transitory embryonic kidneys are similar to those we find in species that evolved before us in the fossil record— jawless fish and reptiles, respectively.”  OCR hyphens in develop- ment and spe- cies joined. ↩

    3. 12c.

      Pages 77–78:  “the descendant inherits the developmental program of its ancestor: that is, all the genes that form ancestral structures.  And development is a very conservative process.  Many structures that form later in development require biochemical "cues" from features that appear earlier.” ↩

    4. 12d.

      Page 80:  “Now, there's no need for a human embryo to have a transitory coat of hair.  After all, it's a cozy 98.6 degrees Fahrenheit in the womb.  Lanugo can be explained only as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development.  Their hair, however, doesn't fall out, but hangs on to become the adult coat.  And, like humans, fetal whales also have lanugo, a remnant of when their ancestors lived on land.” ↩ ↩

  13. 13.

    Dao, Anh H., and Martin G. Netsky.  “Human tails and pseudotails.”  Human Pathology 15(5):449–453.  1984.  Copyright.  Abstract only (PubMed 6373560), in the source library.

    1. 13a.

      Section Abstract:  “A case of a tail in a 2-week-old infant is reported, and findings from a review of 33 previously reported cases of true tails and pseudotails are summarized.” ↩

  14. 14.

    Lu, Frank L. et al.  “The human tail.”  Pediatric Neurology 19(3):230–233.  1998.  Copyright.  Abstract only (PubMed 9806144), in the source library.

    1. 14a.

      Section Abstract:  “In reviewing the literature from 1960 to 1997, 59 cases were described.” ↩

  15. 15.

    Domagała, Z. et al.  “The sequence of lanugo pattern development on the trunk wall in human fetuses.”  Advances in Clinical and Experimental Medicine 26(6):967–972.  2017.  Copyright.  Abstract only (PubMed 29068598), in the source library.  278 fetuses, 69 to 226 days after conception.

    1. 15a.

      Section Abstract, Results:  “The study revealed that the first single hairs occur on the posterior wall of the trunk in the 17th week of fetal life, and on the anterior wall between the 18th and 19th week.”  “It was found that the process of lanugo development was completed on the posterior wall in the 23rd week, and on the surface of the abdomen in the 26th week.” ↩

  16. 16.

    Blackburn, Susan Tucker.  Maternal, Fetal, & Neonatal Physiology: A Clinical Perspective.  Saunders Elsevier, St. Louis.  3rd edition.  2007.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 20, anchored by the index entries Lamellar bodies, formation of, 333 (leaf 353), Laminar flow, 350 (leaf 370) and Lanugo, 531 (leaf 551).

    1. 16a.

      Page 529:  “Lanugo is shed before or after delivery and replaced by shorter, coarser hairs (vellus hairs) from new follicles.” ↩

    2. 16b.

      Page 530:  “Vernix caseosa forms as a superficial fatty film after 17 to 20 weeks.”  “After 36 weeks, the vernix thins and begins to disappear.”  “The vernix has a tendency to accumulate at sites of dense lanugo growth.” ↩ ↩

    3. 16c.

      Page 445:  “Meconium consists of vernix caseosa, lanugo, squamous epithelial cells, occult blood, and”. ↩

  17. 17.

    Darwin, Charles.  The Descent of Man, and Selection in Relation to Sex, vol. 1.  John Murray.  1st edition.  1871.  Public domain.  Darwin Online transcription with page markers, in the source library.

    1. 17a.

      Page 25:  “The fine wool-like hair, or so-called lanugo, with which the human foetus during the sixth month is thickly covered, offers a more curious case.”  Page 26:  “the soles of the feet are quite naked, like the inferior surfaces of all four extremities in most of the lower animals.  As this can hardly be an accidental coincidence, we must consider the woolly covering of the foetus to be the rudimental representative of the first permanent coat of hair in those mammals which are born hairy.” ↩

  18. 18.

    Brauer, Philip R.  Human Embryology: The Ultimate USMLE Step 1 Review.  Hanley & Belfus, Philadelphia.  2003.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 16, anchored by the index entries Fingerprints, 95 (leaf 111) and First arch syndrome, 53, 61 (leaves 69 and 77).

    1. 18a.

      Page 95:  “Vernix caseosa is a culmination of sebaceous gland secretions and dead epidermal cells, and the lanugo hair helps retain it on the outer skin surface.”  The OCR reads IS for is. ↩

  19. 19.

    Bystrova, K.  “Novel mechanism of human fetal growth regulation: a potential role of lanugo, vernix caseosa and a second tactile system of unmyelinated low-threshold C-afferents.”  Medical Hypotheses 72(2):143–146.  2009.  Copyright.  Abstract only (PubMed 19004563), in the source library.

    1. 19a.

      Section Abstract:  “Repeated oscillations of lanugo hairs encased by vernix caseosa during fetal movements in amniotic fluid activate highly sensitive mechanoreceptors connected to unmyelinated C-afferents”. ↩

  20. 20.

    Pavone, Piero et al.  “Congenital generalized hypertrichosis: the skin as a clue to complex malformation syndromes.”  Italian Journal of Pediatrics 41:55.  2015.  CC BY 4.0.  In the source library.  Mirror:  PMC

    1. 20a.

      Section Congenital hypertrichosis lanuginosa:  “In normal individuals, lanugo hairs are often present at birth and tend to gradually disappear over the first months of life.  In affected patients, the lanugo distribution continues to progress, involving the whole body”.  “This form has been reported in patients with abnormalities of chromosome 8q”. ↩

    2. 20b.

      Section Congenital generalized hypertrichosis:  “To date, a clear specific molecular abnormity has not been proved.  A hypothesis has been advanced to explain the typical phenotypic aspect of these patients: an atavistic reversion of a suppressed ancestral gene.” ↩

  21. 21.

    Strumia, R.  “Skin signs in anorexia nervosa.”  Dermato-Endocrinology 1(5):268–270.  2009.  Copyright, Landes Bioscience.  In the source library.  Mirror:  PMC

    1. 21a.

      Section Symptoms due to starvation:  “Lanugo-like body hair is a frequent sign in AN, especially in younger patients.  It presents as fine, downy, pigmented hairs on the back, abdomen and forearms.  It is not a sign of virilization and has been associated with decreased activity of the 5-α-reductase enzyme system, probably due to hypothyroidism.” ↩

  22. 22.

    Center for Academic Research and Training in Anthropogeny.  “Lanugo Hair.”  Matrix of Comparative Anthropogeny.  n.d.  CARTA is a University of California San Diego research center.  Saved to the source library 2026-09-27.

    1. 22a.

      Section Human Uniqueness Compared to Great Apes:  “Interestingly, lanugo hair is also found on some other new-born mammals such as seals and elephants, which like humans, do not have a lot of body hair as adults.”  “In contrast, great apes are born with a considerable amount of regular hair.  Do great apes have lanugo hair in an earlier stage of embryogenesis?”  “No related publications have been added for this topic”. ↩ ↩

  23. 23.

    Coyne, Jerry A.  “Musings on Why Evolution is True (the book).”  Why Evolution Is True.  Jul 19, 2024.  Copyright.  Blog post saved to the source library.

    1. 23a.

      Paragraph 13:  “And I haven’t yet heard of an adaptive explanation for nipples in human males, our wisdom teeth, the developmental sequence of our kidney, or our transitory coat of hair in utero (the “lanugo”), but I’m sure that if you look hard enough on the Internet, you’ll find IDers and creationists showing how these aren’t really “senseless signs of history”, but are actually adaptive.” ↩

  24. 24.

    Romer, Alfred Sherwood, and T. S. Parsons.  The Vertebrate Body.  Saunders College Publishing, Philadelphia.  6th edition.  1986.  Copyright.  Print-disabled archive.org item read by search-inside;  not stored.  Leaf = page + 12, anchored by the index entries Haptodus, skeleton of, 186 (leaf 198) and Hesperocyon, 593 (leaf 605).  This is the passage quoted in van Niekerk 2012, endnote 12.

    1. 24a.

      Page 407:  “It is often stated or implied that these three are distinct kidneys that have succeeded one another phylogenetically as they do embryologically.  However, there is little reason to believe this.  The differences are readily explainable on functional grounds; the three appear to be regionally specialized parts of the original holonephros, which serve different functions.”  The OCR splits How- ever across a line;  joined here. ↩

    2. 24b.

      Page 407:  “Pronephric units in mammals are at best simple structures, without glomeruli and often rudimentary”.  “An actively growing embryo has wastes to excrete.  When kidney tubules are formed anteriorly, they begin the process of urine formation.”  Page 408:  “In amniotes and Chondrichthyes, the pronephros is an exceedingly short-lived structure; it never survives in the adult, and, except for its association with formation of the duct, it hardly deserves a separate name.” ↩ ↩

    3. 24c.

      Page 407:  “Although phylogenetically new, the ureter precedes and even induces the tubules embryologically.” ↩

    4. 24d.

      Page 408:  “All in all, it is perhaps best to confine the term mesonephros solely to the kidney of embryonic amniotes.  The adult anamniote kidney may be termed the opisthonephros, with the understanding that the amniote metanephros is a special variety of the posterior part of this structure.” ↩

    5. 24e.

      Page 405, figure 293:  “E, Metanephros of amniotes: an opisthonephros with a single additional duct, the ureter, draining all tubules.”  The OCR reads £E for E. ↩

  25. 25.

    Menton, David N.  “Vestigial Organs:  Evidence for Evolution?”  Answers in Genesis.  Jul 7, 2014.  Copyright.  Saved to the source library.

    1. 25a.

      Section Vestiges of Embryology:  “There are several structures that function during the development of the embryo and fetus that appear to be no longer used after birth.  Remnants of these once-functional structures persist throughout life.  Such structures perfectly fit the definition of a vestige, but they are not vestiges of evolution.” ↩

  26. 26.

    Wile, Jay L.  “Coyne and Embryonic Development...Wrong AGAIN!”  Proslogion.  May 31, 2010.  Copyright.  Young-earth creationist blog, saved to the source library.

    1. 26a.

      Paragraph 4:  “He seems to think that the only reason a human embryo would need hair is for insulation.” ↩

    2. 26b.

      Paragraph 14:  “Without lanugo, the embryonic skin would not form the protective layer that it needs while in the womb.” ↩

  27. 27.

    Wile, Jay L.  “No, It's Not a Tail!”  Proslogion.  May 21, 2012.  Copyright.  Saved to the source library.  The old address blog.drwile.com/?p=7621 redirects here.

    1. 27a.

      Paragraph 5:  “the caudaul eminence, and it has nothing to do with a tail”.  Paragraph 11:  “As you can see, then, the idea that human embryos have temporary tails during their embryonic development has been thoroughly debunked in the scientific literature.”  caudaul is Wile's spelling. ↩

  28. 28.

    Lamb, Andrew.  “Human tails and fairy tales.”  Creation Ministries International.  Aug 27, 2007.  Copyright.  Saved to the source library.

    1. 28a.

      Paragraph 12:  “Both the TalkOrigins webpage and the original Bar-Meor paper promulgate the false idea that in the womb people have an ‘embryonic tail’.  The correct term for the structure in question is the caudal eminence.” ↩

    2. 28b.

      Paragraph 12:  “But since they do not develop, it is pure evolution-inspired supposition to presume to know what they would develop into, and to label them ‘coccygeal’.” ↩

  29. 29.

    van Niekerk, E.  “Vas deferens.”  Journal of Creation 26(3):60–67.  Dec 2012.  Copyright.  Creation Ministries International web copy, in the source library.  Endnote 12 cites Romer and Parsons 1986 p. 407.

    1. 29a.

      Section Ontogeny and phylogeny:  “It is often stated or implied that these three are distinct kidneys that have succeeded one another phylogenetically [in the creature’s evolutionary history] as they do embryologically.  However, there is little reason to believe this.  The differences are readily explainable on functional grounds; the three appear to be regionally specialized parts of the holonephros, which serve different functions.”  Compare Romer and Parsons p. 407, which reads parts of the original holonephros and has no bracketed gloss. ↩

    2. 29b.

      Section Ontogeny and phylogeny:  “So, a simpler kidney-and-duct system is needed long before it would actually be practically possible to have a fully formed metanephric system.  So the simpler system has nothing to do with evolutionary history, and everything to do with the requirements of the embryo at the early stage.  This is practical design economy once again.”  “Not only embryonic fishes and amphibians (and briefly reptile, bird and mammal embryos) but also adult hagfish and some bony fish species display the pronephros setup.” ↩