THE F(ETAL MEMBRANES OF REPTILES AND BIRDS. 



207 



special 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- 

 layers, and indeed to that part 

 which in Fishes is employed for 

 the yolk-sac. They arise from 

 folds, which grow around the 

 embryo while it is still 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 Fishes, three 

 additional embryonic appen- 

 dages, the amnion, the mem- 

 brcvna serosa (or briefly Derosa), 

 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. 



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

 . a blastoderm of a Chick of 18 hours, after 



BALFOUR. 



In front of the primitive groove, pr, lies the 

 medullary furrow surrounded by the medullary 

 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 amnion 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 



