3 14 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Fig- 5- Transverse section through the anterior part of the head of an embryo 

 belonging to stage F ( x 60 diameters). It shews (i) the ventral curvature of the 

 medullary folds next the head. (2) The absence of mesoblast in the anterior part of 

 the head, hy points to the extreme front end of the alimentary canal. 



Fig. 6. Section through the head of an embryo at a stage intermediate between F 

 and G. x 86 diameters. 



It shews the manner in which the medullary folds of the head unite to form the 

 medullary canal. 



Fig. 7. Longitudinal and vertical section through the tail of an embryo belonging 

 to stage G. 



It shews the direct communication which exists between the neural and alimentary 

 canals. 



The section is not quite parallel to the long axis of the embryo, so that the proto- 

 vertebrae are cut through in its anterior part, and the neural canal passes out of the 

 section anteriorly. 



Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H. 



