ox 'I'lIE FOETAL MI'rMl'.K'AXI'lS OF t'llELOXFA, 21 



iii(lc|)('iulfiitlv i'ufiiK'd vc'sick' which aricfwiinls puts itself in ((tm- 

 niiinicatinii with ilic liiinl-f|'iit hv an iniii'|)cii(l('nl Iv locincil sfalk. 

 The fiii'nres üiven ab<)\e siitticiently wai-ranl us in oouchidin^' that in 

 Clielonia at least, the allantois arises as a d i verti e u I u iii 

 of the hinrl-o'iit and is from tlie first (;o7i t i ii no us witli 

 it. 



III. Laler Stages of the Foetal Membranes. 



hi tiie urecediuii" tuo sections \vc lollowcd sc[i;(r;ncl\- i he L;r<iwth 

 of I lie anuiion aiid (){' I he allantnis uji loa certain <tau('. It will he 

 more e(jn\ »nient to treat the later .stages of these meiidiranes 

 together. As tlie develo[)ment advances, they begin to differ in the 

 two genera (demmys and Trionvx, until, wlien completed, they 

 ])resent important dillcreiices in their struct tu'cs. As those in 

 (demmys ])resent in mv cijiinion more primitive relations, I hegin 

 with tliat species. 



(I. i 'h'Uimijs J(lj)<tH/nl. 



As llie aliantois pushes itself out as a vesicle into the; extra- 

 eml)r\'onic C(elomi<' ca\itv. tlie allantoic hlood-Ncssels ai'e soon found 

 distrihiited in two groups (('ompare f'ig, 27 Tl. \\ .). One (the 

 right) s(M of arteries and veins is ]>laced in that part <d' the \esiele 

 fa<'ing antei"iorly while the otlici" (llie lel'l) set is placerl on the posterioi' 

 external aspect of tluî V(îsic]e. The mannei- of distrihiition of the 

 blood-vessels exerts a consid<'ra1)Ie influence on the fut lu'c shape of 

 the aliantois. 



As the aliantois spi-eads itsell' over the emhrvo as well as over 

 the y<dk in the extra-emla-Noni*; Cd-lomic ca\it\', it assumes a peculiar 

 shape represented in iM"g. ()7 (IM. Virt.). 'l'he vesicle now flattened is 

 divided by two peculiar constrictions into two [)arts of une<pinl sizes. 

 The larger part is again subdivided into two lobes by the posterior set 



