GANOIDEI. 



103 



the meroblastic type more nearly than the segmentation of the 

 frog's egg. The first furrow appears at the formative pole, at 

 which the germinal vesicle was situated. The earlier phases of 

 the segmentation are like those of meroblastic ova, in that the 

 furrows only penetrate for a certain distance into the egg. Eight 

 vertical furrows appear before the first equatorial furrow ; which 

 is somewhat irregular, and situated close to the formative pole. 



In the later stages the vertical furrows extend through the 

 whole egg, and a segmentation cavity appears between the small 

 and the large spheres. The segmentation is thus in the main 



FIG. 50. EMBRYOS OF ACIPENSER VIEWED FROM THE DORSAL SURFACE. 

 (After Salensky.) 



A. Stage before the appearance of the mesoblastic somites. 



B. Stage with five somites. 



.]/;/. medullary groove; bl.p. blastopore ; s.d. segmental duct; Fb. fore-brain; 

 Hb. hind-brain; m.s. mesoblastic somite. 



similar to that of a frog, from which it diverges in the fact that 

 there is a greater difference in size between the small and the 

 large segments. 



In the final stages of the segmentation the cells become 

 distinctly divided into two layers. A layer of small cells is 

 placed at the formative pole, and constitutes the epiblast. The 

 cells composing it are divided, like those of Teleostei, etc., into a 

 superficial epidermic and a deeper nervous layer. The remaining 

 cells constitute the primitive hypoblast (the eventual hypoblast 

 and mesoblast) ; they form a great mass of yolk-cells at the 

 lower pole, and also spread along the roof of the segmentation 

 cavity, on the inner side of the epiblast. 



A process of unsymmetrical invagination now takes place, 

 which is in its essential features exactly similar to that in the 



