COMPARISON OF THE GERMINAL LAYERS. 



285 



process causes however the embryo to cease to lie at the edge of 

 the blastoderm, and while situated at some distance from the 

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

 coalesced lips of the blastopore. The above process is diagram- 

 matically represented in fig. 175 B; while as it actually occurs 



FIG. 174. 



DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH 



EMBRYO. 



Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower 

 layer cells and hypoblast with simple shading. 



ep. epiblast; m. mesoblast; al. alimentary cavity; sg. segmentation cavity; nc. 

 neural canal; ch. notochord; x. point where epiblast 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. 



it is shewn in fig. 30, p. 63. The whole closure of the blastopore 

 in Elasmobranchii is altogether unlike what takes place in Am- 

 phibia, where the blastopore remains as a circular opening which 



