46 



GENERAL GROWTH OF THE EMBRYO. 



become completely closed. The anterior region constitutes the brain ; 

 and in this part slight constrictions, not perceptible in views of the 

 embryo as a transparent object, mark off three vesicles. These 

 vesicles are known as the fore, mid, and hind brain. From the fore- 

 brain there is an outgrowth on each side, the first rudiment of the 

 optic vesicles [op). The tail swellings are still conspicuous. 



The tissues of the body have now become fairly transparent, and 

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



Fig. 28. Views of Elasmobeanch embryos. 



A — F. Pkistiurtts. 



G. and H. Scylliom. 



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

 cavity; es. 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. 1st 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; Jit. heart; m. mouth invagination ; an. anal 

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



F. G. H. Older embryos as opaqiie objects. 



