ELASMOBRANCHII. 



39 



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 differentiation 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, Ijj), a 

 splanchnic layer adjoining the hypoblast, 

 and a somatic layer adjoining the epi- 

 blast. Between these two layers there is 

 soon developed in the region of the head 

 a well-marked cavity (fig. 20 A, jyp) which 

 is subsequently continued into the region 

 of the trunk, and forms the primitive 

 body-cavity, equivalent to the cavity origi- 

 nating as an outgrowth of the archenteron 

 in Amphioxus. The body-cavities of the 

 two sides are at first quite independent. 



Coincidentally with the appearance of 

 differentiation into somatic and splanchnic 

 layers the mesoblast plates become in the 

 region of the trunk partially split by a 



;»/) 



Fig. 21. Transyebsk sec- 

 tion THROUGH THE TRUNK OF AN 

 EMBRYO SLIGHTLY OLDER THAN 

 FIG. 28 E. 



nc. neural canal; pi: pos- 

 terior root of spinal nerve; x. 

 subnotochordal rod; ao. aorta; 

 sc. somatic mesoblast ; sp. 

 splanchnic mesoblast ; mp. 

 muscle-plate; mp'. portion of 

 muscle-plate converted into 

 muscle; Vv. portion of the 

 vertebral plate which will give 

 i-ise to the vertebral bodies; al. 

 alimentary tract. 



