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DEVELOPMENT OF ELASMOBRANCH FISHES. 



the Elasmobranch of epiblast and lower layer cells. The floor of 

 the cavity is, in all, formed of so-called yolk (vide PL 7, fig. 4), 

 which in all forms the main mass of the egg. In the Amphibian 

 the yolk is segmented, and, though it is not segmented in the 

 Elasmobranch, it contains in compensation the nuclei so often 

 mentioned. In all, the sides of the segmentation cavity are 

 formed by lower layer cells. In the Amphibian the sides are 



FIG. i. 



Diagrammatic longitudinal sections of an Elnsmobranch 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 segmentation cavity in the middle of the 

 lower layer cells. 



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

 tinctly formed, and in which the alimentary slit has appeared. The segmentation 

 cavity is still represented as being present, though by this stage it has in reality 

 disappeared. 



C. Older blastoderm with embryo in which neural canal has become formed, and 

 is continuous posteriorly with alimentary canal. The notochord, though shaded like 

 mesoblast, belongs properly to the hypoblast. 



