ELASMOBRANCHII. 



45 



cells of the non-embryonic part of the blastoderm. It results 

 from the above mode of development of the mesoblast, that it 

 may be described as arising in the form of a pair of solid out- 

 growths of the wall of the alimentary tract ; which differ from the 

 mesoblastic outgrowths of the wall of the archenteron in Amphi- 

 oxus in not containing a prolongation of the alimentary cavity. 



A general idea of the structure of the blastoderm at this 

 stage may be gathered from the diagram representing a longi- 



FIG. 19. 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 continu- 

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



A. Section of young blastoderm, with segmentation cavity enclosed in 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 the neural canal has become formed, 

 and is continuous posteriorly with the alimentary canal. The notochord, though 

 shaded like mesoblast, belongs properly to the hypoblast. 



