154 



FCETAL MEMBRANES. 



Foetal Membranes. 



The Reptilia, Aves and Mammalia are distinguished from the 

 Ichthyopsida by the possession of certain provisional foetal mem- 

 branes, known as the amnion and allantois. 



As the mode of development of these membranes may be most 

 conveniently studied in the chick, I have selected this type for their 

 detailed description. 



The Amnion. The amnion is a peculiar sack which envelopes, 

 and protects the embryo. 



At the end of the first day of incubation, when the cleavage of 

 the mesoblast has somewhat advanced, there appears, a little way in 

 front of the semilunar head-fold, a second fold (fig. 102, also fig, 

 121, C, «/"and fig. 122 J.m), running more or less parallel or rather 

 concentric with the first and not unlike it in general appearance, 

 though differing widely from it in nature. This second fold gives rise 

 to the amnion, and is limited entirely to the somatopleure. Rising 

 up as a semilunar fold with its concavity directed towards the 

 embryo (fig. 121 C, af), as it increases in height it is gradually 



■»/ 



vt 



•' f/(/^ 



-pp 



Fig. 121. 



Fig. 121 A to N forms a series of purely diagrammatic representations intro- 

 duced to facilitate the comprehension of the manner in which the body of the embryo 

 is formed, and of the various relations of the yolk-sack, amnion, and allantois. 



In all vt is the vitelline membrane, placed, for convenience sake, at some distance 

 from its contents, and represented as persisting in the later stages; in reality it is 

 in direct contact with the blastoderm or yolk, and early ceases to have a separate 

 existence. In all e indicates the embryo proper; /jp *^6 general pleuro-peritoneal 

 space with its extension between the membranes ; af the folds of the amnion ; a the 



