File:The descent of the primates; lectures delivered on the occasion of the sesquicentennial celebration of Princeton university (1897) (14766416861).jpg

Identifier: descentofprimate00hubr (find matches)
Title: The descent of the primates; lectures delivered on the occasion of the sesquicentennial celebration of Princeton university
Year: 1897 (1890s)
Authors: Hubrecht, A. A. W, (Ambrosius Arnold Willem. 1853-1915
Subjects: Primates
Publisher: New York, C. Scribner's sons
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

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e heard Minot affirm.They do not tell us how the amnion can pos-sibly have developed phylogenetically. In thetraditional cases we see it arise as a fold, whichslowly and gradually encloses the embryo. Andonly ivhen the closure has become final is theamnion effective. What, then, were here the in-cipient stages? All this appears in another light when wetrust to the exceptional cases above mentionedto guide us in determining the phylogeny of theamnion. We can then start from the much morereasonable basis that the amnion at its earliestappearance m%tst have heen a closed sac. Onlyon this supposition can it be understood that itwas of high selective significance from the veryfirst. It was modified only gradually,— one ofthe modifications being this, that it did not anylonger arise so very early in individual develop-ment, but only later by means of the foldsalluded to. Now let me emphasize that at thepresent day we do find this very same develop-ment of the amnion as a closed sac in the
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M ri 00 T- -1 rt p. CJ C </) > »■ , , rt rt ^ o OJ r^ .^ U. ^ Ti T-; r. U u o C/0 ;_J T3 t3 ,.^ ^ -^ T3 > ^ rt C/^ T3 a .^^ <v </5 rS O C/3 X> >> ri O n c n IT) rt n = o O =5I/O THE DESCENT OF THE PKIMATES 37 above-named representatives of four differentorders of mammals : Cavia (guinea-pig), Eriiia-ceus (hedgehog), Pteropus (bat, flying-fox), IIo?no(man). This will suffice to convince you thatas far as placental mammals go, the statementthat the amnion arises as an upward-growingfold, which finally encapsules the embryo, doesnot find universal application. There is indeedan equal chance of the other mode of formationhaving been the original one. My own choiceis fixed upon the latter hypothesis because inthe Amphibia, from which I suppose the earliestplacental mammals to have been derived, wefind arrangements that appear to explain theearliest origin of the amnion in the way hereadvocated. There is, moreover, no difficulty in

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