290 



MAMMALIAN GASTRULA. 



quently to the formation of the blastodermic vesicle receive a 

 satisfactory explanation. 



The whole of the blastodermic vesicle, except the embryonic 

 area, represents the yolk-sack, and the growth of the hypoblast 

 and then of the mesoblast round its inner wall represents the 



FIG. 177. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. 



ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. neurenteric canal ; hy. hypo- 

 blast ; p.a.g. post-anal gut ; pr. remains of primitive streak folded in on the ventral 

 side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. peri- 

 visceral cavity am. amnion; so. somatopleure ; sp. splanchnopleure. 



corresponding growths in the Sauropsida. As in the Sauropsida 

 it becomes constricted off from the embryo, and the splanchno- 

 pleuric stalk of the sack opens into the ileum in the usual way. 



FIG. 178. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY 



FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) 

 ep. epiblast; hy. primary hypoblast; bp. Van Beneden's so-called blastopore. 

 The shading of the epiblast and hypoblast is diagrammatic. 



