282 



THE GASTRULA OF ELASMOBRANCHII. 



in the Amphibian embryo. This difference no doubt depends 

 upon the greater quantity of yolk in the Elasmobranch ovum, 

 and a similar distribution of the lower layer cells is found in 

 Acipenser and in Petromyzon. 



In the next stage for the Elasmobranch (fig. 173 and 174 B) 

 and for the Amphibian (fig. 170 C) or better still Petromyzon 



FIG. 173. THREE DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH AN 

 IDEAL TYPE OF VERTEBRATE EMBRYO INTERMEDIATE IN THE MODE OF FOR- 

 MATION OF ITS LAYERS BETWEEN AMPHIBIA OR PETROMYZON AND ELASMO- 

 BRANCHII. 



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

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



(fig. 171) the agreement between the three types is again very 

 close. For a small arc (x) of the edge of the blastoderm the 

 epiblast and hypoblast become continuous, while at all other 



