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Are the membranes present between the fetal fingers and toes a remainder of the phylogenetic evolution, or just a way organs do grow most easily?

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  • $\begingroup$ It seems difficult for me to define "easily" in the context of organ development. In any case, the way organs develop is mostly inherited from our ancestors, so "it's easy to do so" and "it's inherited" are not necessarily contradictory statements. $\endgroup$ – bli Mar 9 '16 at 15:19
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Short answer
The transient membranes between the fingers and toes in embryos are a myth.

Background
The presence of transient membranes in developing embryos supposedly points to our fishy evolutionary origins. However, I couldn't find any credible sources to support this myth. In fact, I couldn't find any source mentioning the presence of webbing between the fingers and toes at any stage during development.

To support this lack of evidence, I have posted several photos of embryos and fetuses in various developmental stages. None show any signs of the "swimming webs". Note that the age mentioned is the developmental age, counted from the moment of conception (add 2 weeks to have gestational age):

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Fig. 1. Five-week embryo. source: Palm Reading Perspectives

7wk
Fig. 2. Seven-week embryo. source: Wikipedia

10wk
Fig. 3. Eight-week embryo. source: WemMD

14wk
Fig. 4. Fourteen-week foetus. source: WemMD

18wk
Fig. 5. Eighteen-week foetus. source: WemMD

Fig. 2 may seem to show some webbing, but if we look at some close-ups of the hand and feet at week 7 in Fig. 6 below, we can see that there is no real webbing; fingers and toes are simply budding from the primordial hand and foot structures.

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Fig. 6. Developmental stages of the hand and feet around week 7. source: I am Pregnant

Note that an embryo is called a fetus only after the first trimester, i.e. after some 12 weeks. There is certainly not any sign of webbings in healthy fetuses.

Also note that webbings may be the result of birth defects, such as syndactyly and pseudosyndactyly (Fig. 7):

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Fig. 7. Infants with Syndactyly (left) and Pseudosyndactyly (right). sources: Wikipedia & University of Columbia

Original photographic image source (Figs 1 & 5)*
- Nilsson, Ett Barn Blir Till

* Credits go to rg255

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  • $\begingroup$ Figs. 1 + 5 at least come from Lennart Nilsson's Ett Barn Blir Till (ISBN: 9789174242751) - just thought that deserves proper credit because it's a beautiful book (and has English versions I think) :) $\endgroup$ – rg255 Mar 22 '16 at 11:44
  • $\begingroup$ @rg255 - thanks! - ref added + acknowledgement :-) $\endgroup$ – AliceD Mar 22 '16 at 12:31
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    $\begingroup$ There is some "webbing" in very early stages. These cells between the digits undergo apoptosis. At least that's what I have read in the dev bio textbooks. $\endgroup$ – WYSIWYG Mar 23 '16 at 7:12
  • $\begingroup$ @WYSIWYG - yes, that's what I thought too. I just couldn't find it. I'll look into some text books and try to find out. Thanks for this. $\endgroup$ – AliceD Mar 23 '16 at 7:20
  • $\begingroup$ Thanks. So (if there's apoptosis of the cells between fingers) the second option (it is the easier way to "grow a hand) is correct, isn't it (but yeah, I understand your point, why no "membranes" exist? $\endgroup$ – Probably Mar 23 '16 at 16:03

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