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- 

 brana 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. 



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 Hue running concentric 

 with it, the anterior fold of the aranion. 



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 



