184 



A. 



THE THIRD DAY. 

 Fm. 63. 



[CHAP. 



B. 



Two LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EM- 

 BRYO CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS. 

 A AT THE BEGINNING OF THE THIRD DAY; B AT THE 

 MIDDLE OF THE THIRD DAY. (After Dobrynin.) 



t. the tail ; m. the mesoblast ; cc f . the epiblast ; x". the neural 

 , canal ; Dd. the dorsal wall of the hindgut ; SO. somato- 

 pleure ; Spl. splanchnopleure ; u. the mesoblast of the 

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

 pleuroperitoneal cavity ; Df. 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- 

 dseum) ; 6f. 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 



