i fja \i. MEM BE LNfcfl OF B \M HIKDS. 



207 



:il ru'LT t-n\. !"]>- (embryonic or I'M til m>-inl>ranes). Some of 

 them, according to tln-ir origin, are to b- ivferr, d to* 

 4-mhrvMiic area of the germ- 

 LayerS, and indeed to that p;irt 

 which in l-'i-he- is employ d for 

 the yolk sac. They arise from 

 folds, which grow around the 

 < inbryo while it is still small, 

 and furnish a double envelope 

 for it. 



The egg- envelopes (embryonic 

 membram s) of Reptiles and 

 Birds, which exhibit almost 

 identical conditions, and the 

 consideration of which we shall 

 take up first, are more simply 

 con.-t it ut d than those of Mam- 

 mals. In the case of the form, r 

 there are associated with the 

 yolk-sac, in the possession of 

 which they agree with the 

 Amphibia and r"i>lu-. three 

 additional < mbryonic app n- 

 dage>. the mitiiion, the mem- 

 1>rnmt serosa ( r briefly *'>rosa), 

 and the alluntoix. They are 

 [.artly laid down at an early 

 d. at the time when the 

 embryonic body is converted 



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

 stricted oil' fiorn the yolk-sac. 



The Chick shall again serve a> a ba>is for our description. 



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

 a blastoderm of a Chick of 18 hours, after 

 15 \i FOUR. 



In front of the primitive groove, pr, lies the 

 medullary furrow surrounded by the medullary 

 folda. Immediately in front of these one see* 

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

 it a second curvod line runuini; 

 with it, the anterior fM of the amnion. 



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



The amiivon is a structure tin- aj>i>raranc- of which i> recognisable 

 remarkably early in the Chick. At the time \\hen one recognises 

 the semicircular bead-fold at the ant rior end of the incipient rinbrvo 

 (tig. 1:M). by the growth of which the he;<l of the rinhryo iv marked 

 oil', there is already pieM-nt, at a short distance from it. a .>econd fold 

 running parallel to it. This is the ant- of the amni 



