COMPAlilSOX OF THE GERMINAL LAYERS. 237 



travels forwards. The process causes however the embryo to cease 

 to lie at the edge of the blastoderm, and while situated at some 



<^-^ 



/^ 



\, 



Fig. 174. Diagbammatic LONGiTtrniNAL sections of an Elasmobrakch embryo. 



Epihlast without shading. Mesohlast black with clear outlines to the cells. Loicer 

 layer cells and hypoblast with simple shading. 



ep. epihlast; m. mesoblast; al. alimentary cavity; sf). segmentation cavity; vc. 

 neural canal; ch. notochord; x. point where epihlast and hypoblast become continuous 

 at the posterior end of the embryo; n. nuclei of yolk. 



A. Section of young blastoderm, with the segmentation cavity enclosed in the lower 

 layer cells (primitive hypoblast). 



B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly 

 formed, and in which the alimentary cavity has appeared. The segmentation cavity is 

 still represented, though by this stage it has in reality disappeared. 



C. Older blastoderm with embryo in which the neural canal is formed, and is 

 continuous posteriorly with the alimentary canal. The notochord, though shaded 

 like mesoblast, belongs properly to the hypoblast. 



distance from the edge, to be connected with it by a linear streak, 

 representing the coalesced lips of the blastopore. The above pro- 

 cess is diagrammatically represented in fig. 175 B ; while as it actually 

 occurs it is shewn in fig. 30, p. 52. The whole closure of the blasto- 

 pore in Elasmobranchii is altogether unlike what takes place in 

 Amphibia, where the blastopore remains as a circular opening which 

 gradually narrows till it becomes completely enveloped in the 

 medullary folds (fig. 175 A). 



On the formation of the neurenteric canal the body of the embryo 

 Elasmobranch becomes gradually folded off from the yolk, which, 

 owing to its great size, forms a large sack appended to the ventral 

 side of the body. The part of the somatopleure, which grows round 



