THE FCETAL MEMBRANES OP REPTILES AND BIRDS. 



207 



«pecial egg-envelopes (embryonic or foetal membranes). Some of 

 them, according to their origin, are to be referred to the extra- 

 embryonic area of the germ- 

 iayers, and indeed to that part 

 which in Kshes is employed for 

 the yolk-sac. They arise from 

 folds, which grow around the 

 embryo while it is stiU small, 

 and furnish a double envelope 

 for it. 



The egg- envelopes (embryonic 

 membranes) of Reptiles and 

 -Birds, which exhibit almost 

 identical conditions, and the 

 ■consideration of which we shall 

 take up first, are more simply 

 ■constituted than those of Mam- 

 mals. In the case of the former 

 there are associated with the 

 yolk-sac, in the possession of 

 -which they agree with the 

 Amphibia and Kshes, three 

 ■additional embryonic appen- 

 ■dages, the a/mnion, the mem- 

 hra/iia serosa (or briefly serosa), 

 ■and the allantois. They are 

 partly laid down at an early 

 period, at the time when the 

 embryonic body is converted 



into tubes by the infolding of the germ-layers and is thereby con 

 stricted off from the yolk-sac. 



The Chick shall again serve as a basis for our description. 



\ 



V. 



y 



Fig. 124.— Surface-view of the pellucid area of 

 a blastoderm of a Chick of 18 hours, after 

 Balfotjb. 



In front of the primitlTe groove, ^r, lies >the 

 medullary furrow surrounded bj the medullarj 

 folds. Immediately in front of these one sees 

 a curved line, the head-fold, and in front of 

 it a second curved line running concentric 

 with it, the anterior fold of the amnion. 



1. The Amnion, the Serosa, and the Yolk-Sac. 



The a/mnwn is a structure the appearance of which is recognisable 

 remarkably early in the Chick. At the time when one recognises 

 the semicircular head-fold at the anterior end of the incipient embryo 

 (fig. 124), by the growth of which the head of the embryo is marked 

 off, there is already present, at a short distance from it, a second fold 

 running parallel to it. This is the anterior fold of the amnion, a 



