GAXOIDEI. 



85 



In the later stages the vertical furrows extend through the whole 

 and a segmentation cavity appears between the small and the 

 large spheres." The segmentation is thus in the main similar to that 



a frog, from which it diverges in the fact that there is a greater 

 difference in size between the small and the large segments. 



of 



FIG. 50. EMI;I;YOS OF ACIPENSEK VIEWED FROM THE DOBSAL SURFACE. 



(After Salensky.) 



A. Stage before the appearance of the mesoblastic somites. 



B. Stage with live somites. 



My. medullary groove; II. p. blastopore; s.d. segmental duct; Fb. fore-brain; 

 HI), hind-brain; m.s. mesoblastic somite. 



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 pri- 

 mitive 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 vmsymmetrical invagination now takes place, which 

 is in its essential features exactly similar to that in the frog or the 

 lamprey, and I must refer the reader for the details of the process 

 to the chapter oil the Amphibia. The edge of the cap of epiblast 

 forms an equatorial line. For the greater extent of this line the 

 epiblast cells grow over the hypoblast, as in an epibolic gastrula, but 

 for a small arc they are inflected. At the inflected edge an invagi- 

 nation of cells takes place, underneath the epiblast, towards the 

 segmentation cavity, and gives rise to the dorsal wall of the me- 

 senteron and the main part of the dorsal mesoblast. The slit below 

 the invaginated layer gradually dilates to form the alimentary cavity ; 

 the ventral wall of which is at first formed of yolk-cells. The epi- 

 blast along the line of the invaginated cells soon becomes thickened, 

 and forms a medullary plate, which is not very distinct in surface 

 views. The cephalic extremity of this plate, which is furthest re- 

 moved from the edge, dilates, and the medullary plate then assumes 

 n spatula form (fig. oO A, ' 



