184 



THE THIRD DAY. 

 Fig. 63. 



[<;11AP. 



B. 



Spf 



Two Longitudinal Sections op the Tail-end op an Em- 

 bryo Chick to shew the Origin op the Allantois. 

 A AT the beginning op the Third Day ; B at the 

 MIDDLE op the Third Day. (After Dobrynin.) 



t. the tail ; m. the mesoblaat ; .•)/. the epiblast ; a/', the neural 

 canal ; Dd. the dorsal wall of the hiudgut ; SO. somato- 

 pleure ; &pl. splanchnopleure ; u. the mesoblast of the 

 splanchnopleure carrying the vessels of the yolk-sac ; pp. 

 pleuroperitoneal cavity ; Z>/. the epithelium lining the 

 pleuroperitoneal cavity ; All. the commencing allantois ; 

 w. projection formed by anterior and posterior divisions of 

 the primitive streak ; y. hypoblast which will form the 

 ventral wall of the hindgut ; v. anal invagination (procto- 

 deeum); O. cloaca. 



day project beyond this point ; but on the fourth day 

 begins to pass out beyond the body of the chick, along 

 the as yet wide space between the. splanchnic and soma- 

 tic stalks of the embryo, on its way to the space between 

 the external and internal folds of the amnion, which it 

 will be remembered, is directly continuous with the 

 pleuroperitoneal cavity (Fig. 9 K). In this space it 



