EMBRYONIC MEMBRANES 



6si 



which must enclose the yolk in the course of the development, 

 and it also makes necessary the provision of special apparatus for 

 the retention of water, for respiration, and for storing excreta. 

 These things are accomplished in the processes of the folding off 

 of the embryo and the formation of the embryonic membranes 

 which are found in reptiles, birds, and mammals (Fig. 508). 

 The blastoderm continues to extend until, at a late period in 



a.o. 



a.v. 



a.p. ""■"""" 



mes.L 



n.J. 

 nch. 

 'pr.gr. 



^■a:^^-&^' 



Fig. 509. — A diagrammatic dorsal view of the blastoderm of a fowl's eg% towards 

 the end of the first day (between Figs. 3 and 4 in Fig. 503). 



a.o., Area opaca ; a.p., area pellucida ; a.p'., part of area pellucida into which mesoderm has not penetrated 

 (proanmion) ; a.v., area vasculosa ; h.f., head fold ; mes.L, anterior limit of mesoderm ; n.f., neural 

 folds ; nch., notochord ; pr.gr., primitive groove. 



incubation, it has completely enclosed the yolk. As development 

 proceeds, there appears in the area pellucida a furrow that 

 surrounds a central region in which the embryo is forming. The 

 furrow begins in front, in a region called the proamnion, which 

 does not yet contain mesoderm, as the crescentic head fold, 

 and is completed by a tail fold behind, which joins the head 

 fold at the sides. These folds deepen, and pinch off the embryo 

 proper from the rest of the blastoderm and the yolk around 

 which the latter is growing (Fig. 503, 5). The splanchnopleure, 

 as it folds inwards (Fig. 511, B), comes to form in the body 



