58 



FORMATION OF THE LAYERS. 



interest. An answer to it lias already been attempted from a 

 general point of view in my paper* entitled ' The Comparison of 

 the early stages of development in Vertebrates'; but the subject 

 may be conveniently treated here in a special manner for 

 Elasmobranch embryos. 



In the wood-cut, fig. 1 A, B, C^ are represented three dia- 

 grammatic longitudinal sections of an Elasmobranch embryo. 



Fig. 1. 



Diagrammatic longitudinal sections of an Elasmobranch embryo. 



Epihlcist withont shading. Mesoblast black with clear outlines to the cells. 

 Lower layer cells and hypoblast with simple shading. 



ep. epiblast. m. mesoblast. al. alimentary cavity. scj. segmentation 

 cavity, nc. neural canal, ch. notochord. x. point where epiblast and hypo- 

 blast 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 

 distinctly formed, and in which the alimentary slit has appeared. The seg- 

 mentation 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. 



1 Loc. cit. 



2 This figure, together with fig. 2 and 3, are reproduced from my paper upon 

 the comparison of the early stages of development in vertebrates. 



