ELASMOBRANCHII. 37 



embryo. They soon however grow forwards as two' lateral ridges, 

 attached to the hypoblast, one on each side of the medullary 

 groove (fig. 18 A, in). These ridges become separate from the hypo- 

 blast, and form two plates, thinner in front than behind ; but still 

 continuous at r the edge of the blastoderm with the uudifferentiated 

 cells of the lip of the blastopore, and laterally with the lower layer 

 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 outgrowths of the 

 wall of the alimentary tract ; which differ from the mesoblastic out- 

 growths of the wall of the archeuteron in Amphioxus in not contain- 

 ing a prolongation of the alimentary cavitv. 



A general idea of the structure of the blastoderm at this stage 

 may be gathered from the diagram representing a longitudinal section 

 through the embryo (fig. 19 B). In this figure the epiblast is repre- 



FlG. 19. DlAGEAJIHATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH EMBKYO. 



Epiblast without shading, Mewblast black with clear outlines to the cells. Loiver 

 layer cells and hypoblast with simple shading. 



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

 neural canal; di. notochord; .r. point where epiblast and hypoblast become continuous 

 at the posterior end of the embryo; H. 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 distinctly 

 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. 



