COMPARISON OF THE GERMINAL LAYERS. 



231 



to deal comparatively with the formation of the layers in the Verte- 

 brata. It may be laid down as a general law, which holds very 

 accurately for the Vertebrata, that in eggs in which the distribution of 

 food-yolk is not uniform, the size of the cells resulting from segmen- 

 tation is proportional to the quantity of food-material they contain. 

 In accordance with this law the cells of the Amphibian ovum are of 

 unequal size even at the close of segmentation. They may roughly 

 be divided into two categories, viz. the smaller cells of the upper 

 pole and the larger of the lower (fig. 170 A). The segmentation 

 cavity {sg) lies between the two, but is unsymmetrically placed near 

 the upper pole of the e^g, owing to the large bulk of the ventrallv 

 placed yolk -segments. In the inequality of the cells at the close of 

 segmentation the Amphibia stand in contrast with Amphioxus. The 



Fig. 170. Diagkammatic longitudinal sections through the embryo op Bom- 



BINATOR AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS. (Modifiei 



from Gotte.) 



ep. epiblast; m. dorsal mesoLlast; m'. ventral mesoblaat; /i?/. hypoblast; yk. yolk; 

 X. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore ; 

 al. mesenteron; k//. segmentation cavity. 



