GO 



FOKMATION OF THE LAYERS. 



Diagrammatic longitudinal sections of embryo, wliicli develops in the same 

 manner as the Elasmobranch embryo, but in which the ovum contains far 

 less food-yolk than is the case with the Elasmobranch ovum. 

 Epiblast without shading. Mesohlast black with clear outHnes to the cells. 



Loiver layer cells and hypoblast with simple shading. 



ep. epiblast. m. mesoblast. hy. hypoblast. sg. segmentation cavity. 



al. alimentary cavity, nc. neural canal, hf. head-fold. n. nuclei of the yolk. 

 The stages A, B and G are the same as in figure 1. 



with fig. 3 A. In all there is present a segmentation cavity 

 situated not centrally but near the surface of the egg. The 

 roof of the cavity is thin in all, being composed in the 

 Amphibian of epiblast alone, and in the Elasmobranch of 

 epiblast and lower layer cells. The floor of the cavity is, in 



Fig. 3. 



'L 



