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 



Air- 



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. 



R 



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. 



