GENERAL GROWTH OF THE EMBRYO. 



The tissues of the body have now become fairly transparent, 

 and there may be seen at the sides of the body seventeen 

 mesoblastic somites. The notochord, which was formed long 



jug 



FIG. 28. VIEWS OF ELASMOBRANCH EMBRYOS. 

 A F. PRISTIURUS. G. and H. SCYLLIUM. 



A. A blastoderm before the formation of the medullary plate, sc. segmentation 

 cavity ; cs. embryonic swelling. 



B. A somewhat older blastoderm in which the medullary groove has been es- 

 tablished, mg. medullary groove. 



C. An embryo from the dorsal surface, as an opaque object, after the medullary 

 groove has become posteriorly converted into a tube. mg. medullary groove : the 

 reference line points very nearly to the junction between the open medullary groove 

 with the medullary tube ; h. cephalic plate ; ts. tail swelling. 



D. Side view of a somewhat older embryo as a transparent object, ch. notochord ; 

 op. optic vesicle ; I.v.c. ist visceral cleft; al. alimentary tract ; so.s. stalk connecting 

 the yolk-sack with the embryo. 



E. Side view of an older embryo as a transparent object, mp. muscle-plates ; 

 au.v. auditory vesicle ; vc. visceral cleft ; lit. heart ; in. mouth invagination ; an. anal 

 diverticulum ; al.v. posterior vesicle of post-anal gut. 



F. G. II. Older embryos as opaque objects. 



