COMPARISON OF THE GERMINAL LAYERS. 



235 



epithelium is formed directly from the existing cells, while in Elasmo- 

 branchii it is derived from cells formed around the nuclei of the yolk. 



Fio. 178. Three diagrammatic longitudinal sections through an ideal type 

 OF Vertebrate embryo intermediate in the mode of formation of its layers 

 BETWEEN Amphibia or Petromyzon and Elasmobranchii. 



sq. segmentation cavity; ep. epiblast; m. mesoblast; hy. hypoblast; nc. neural 

 canal ; al. mesenteron ; n. nuclei of the yolk. 



As in the earlier stage, so in the present one, the anatomical 

 relations of the yolk to the blastoderm in the one case (Elasmo- 

 branchii) are nearly identical with those of the yolk-cells to the 

 blastoderm in the other (Amphibia). 



The main features in which the two embryos differ, during the 

 stage under consideration, arise from the same cause as the solitary 

 point of difference during the preceding stage. 



In Amphibia the alimentary cavity is formed coincidently with a 

 true ingrowth of cells from the point where epiblast and hypoblast 

 become continuous ; and from this ingrowth the dorsal wall of the 

 alimentary cavity is formed. The same ingrowth causes the oblitera- 

 tion of the segmentation cavity. 



