CHAPTER XI 

 EXTRA-EMBRYONIC MEMBRANES 



THE FOLDING OFF OF THE BODY OF THE EMBRYO; THE ESTABLISH- 

 MENT OF THE YOLK-SAC AND THE DELIMITATION OF THE 

 EMBRYONIC GUT; THE AMNION AND THE SEROSA; THE 

 ALLANTOIS. 



The Folding off of the Body of the Embryo. In bird embryos 

 the somatopleure and splanchnopleure extend over the yolk 

 peripherally, beyond the region where the body of the embryo 

 is being formed. Distal to the body of the embryo the layers 

 are termed extra-embryonic. At first the body of the chick has 

 no definite boundaries and consequently embryonic and extra- 

 embryonic layers are directly continuous without there being 

 any definite boundary at which we may say one ends and the 

 other begins. As the body of the embryo takes form, a series 

 of folds develop about it, undercut it, and finally nearly separate 

 it from the yolk. The folds which thus definitely establish the 

 boundaries between intra-embryonic and extra-embryonic 

 regions are known as the limiting body folds or simply the body 

 folds. 



The first of the body folds to appear is the fold which marks 

 the boundary of the head. By the end of the first day of incu- 

 bation the head has grown anteriorly and the fold originally 

 bounding it appears to have undercut and separated it anteriorly 

 from the blastoderm (Figs. 15 and 17, ). The cephalic limit- 

 ing fold at this stage is crescentic, concave caudally. As this 

 fold continues to progress caudad, its posterior extremities 

 become continuous with folds which develop along either side of 

 the embryo. Because of the fact that these folds bound 

 the body of the embryo laterally, they are known as the lateral 

 body folds (lateral limiting sulci). The lateral body folds, at 

 first shallow (Fig. 28, D) become deeper, undercutting the body 

 of the embryo from either side and further separating it from 

 the yolk (Fig. 36, E and Fig. 30). 



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