7iS 



PHYLUM CHORDATA : CLASS REPTlLlA 



these folds is double ; the inner limbs unite to form " the true amnion " ; 

 the outer limbs unite to form "the false amnion," "serous membrane," 

 or subzonal membrane. The cavity bounded by the true amnion 

 contains an amniotic fluid bathing the outer surface of the embryo ; 

 the cavity between the true and the false amnion is lined by mesoderm, 

 and is continuous with the pleuro-peritoneal or body cavity of the 

 embryo. The amniotic folds extend not only over the embryo, but 

 ventrally around the yolk-sac, which they completely invest. 



(b) The Allantois. — While the amnion is being formed, a sac grows 

 out from the hind end of the embryonic gut. This is the allantois 



lined internally by endoderm, externally 

 by mesoderm. It rapidly insinuates itself 

 between the two limbs of the amnion, 

 eventually surrounding both embryo and 

 yolk-sac. 



The amnion is a protective membrane, 

 forming a kind of water-bag around the 

 embryo. 



The allantoic sac is vascular, and has 

 respiratory and perhaps also some yolk- 

 absorbing functions. It seems to be 

 homologous with the outgrowth which 

 forms the cloacal bladder of Amphibians ; 

 it has been called " a precociously de- 

 veloped urinary bladder." 



Before the amnion is developed, the 

 heavy head end of the embr\'o has already 

 sunk into a depression (in Lizards, Chel- 

 onians, Birds (?), and Mammals), and is 

 -S.Z.m surrounded by a modification of the head 

 fold termed the pro-amnion. This does 

 not include any mesoderm, and is after- 

 wards replaced by the amnion. 



Hints of a placenta before Mammals. — 

 As will be explained afterwards, the 

 placenta, which characterises most Mam- 

 mals, is an organic connection between 



Fig. 426. — Origin of amnion and 

 allantois. — After Balfour. 



1. Rise of amniotic folds {a.f.) around embryo(f.) ; 



p.p., pleuro-peritoneal cavity ; y., yolk. 



2. Further growth of amniotic folds {a.f.) over 



embryo and around yolk. 



3. Fusion of amniotic folds above embryo ; a. p., 



amnion proper; s.z.m., subzonal membrane; 

 y..'>., yolk-sac. 



4. Outgrowth of allantois (al.) ; amniotic cavity 



{a.c.) ; h., head end ; /., tail end. 



5. Complete enclosure and reduction of yolk-sac 



(y.s.) ; s.z.m., subzonal membrane ; a. p., 

 amnion proper ; al., allantois ; g., gut of 

 embrvo. 



al^ 



