ELASMOBRANCHII. 47 



By this stage the three layers of the body, the epiblast, 

 mesoblast, and hypoblast, have become definitely established. 

 The further history of these layers may now be briefly traced. 



Epiblast. While the greater part of the epiblast becomes 

 converted into the external epidermis, from which involutions 

 give rise to the olfactory and auditory pits, the lens of the eye, 

 the mouth cavity, and anus, the part of it lining the medullary 

 groove becomes converted into the central nervous system and 

 optic cup. The medullary groove is at first continued to the 

 front end of the medullary plate ; but the anterior part of this 

 plate soon enlarges, and the whole plate assumes a spatula form 

 (fig. 28 C, h, and fig. 20 A and B). The enlarged part becomes 

 converted into the brain, and may be called the cephalic plate. 



The posterior part of the canal deepens much more rapidly 

 than the rest (fig. 20 C), and the medullary folds unite dorsally 

 and convert the posterior end of the medullary groove into a 

 closed canal, while the groove is still widely open elsewhere. 

 The medullary canal does not end blindly behind, but simply 

 forms a tube not closed at either extremity. The importance of 

 this fact will appear later. 



Shortly after the medullary folds have met behind the whole 

 canal becomes closed in. This occurs in the usual way by the 

 junction and coalescence of the medullary folds. In the course 

 of the closing of the medullary groove the edges of the cephalic 

 plate, which have at first a ventral curvature, become bent up in 

 the normal manner, and enclose the dilated cephalic portion 

 of the medullary canal. The closing of the medullary canal 

 takes place earlier in the head and neck than in the back. 



An anterior pore at the front end of the canal, like that in 

 Amphioxus and the Ascidians, is not found. The further differ- 

 entiation of the central nervous system is described in a special 

 chapter: it may however here be stated that the walls of the 

 medullary canal give rise not only to the central nervous system 

 but to the peripheral also. 



Mesoblast. The mesoblast was left as two lateral plates 

 continuous behind with the undifferentiated cells of the caudal 

 swellings. 



The cells composing them become arranged in two layers 

 (fig. 20 C, lp}, a splanchnic layer adjoining the hypoblast, and a 



