300 DEVELOPMENT OF ELASMOBRANCH FISHES. 



time apparent at this stage, but becomes more conspicuous 

 during the succeeding ones, and attains its maximum in stage F 

 (PI. 10, fig. 5). in which it might almost be supposed that the 

 edges of the cephalic plate were about to grow downwards and 

 meet on the ventral side of the embryo. 



In the stages subsequent to D the posterior part of the 

 canal deepens much more rapidly than the rest (vide PI. 10, 

 fig. 4, taken from the posterior end of an embryo but slightly 

 younger than F), and the medullary folds unite and convert 

 the posterior end of the medullary groove into a closed canal 

 (PL 8, fig. F), while the groove is still widely open elsewhere 1 . 

 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. 



In a stage but slightly subsequent to F nearly the whole of 

 the medullary canal becomes formed. 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 lose their ventral curvature and become bent 

 up in the normal manner (vide PI. 10, fig. 6, a section talcen 

 through the posterior part of the cephalic plate), and the en- 

 larged plate merely serves to enclose a 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. The 

 anterior end of the canal becomes closed and does not remain 

 open like the posterior end. 



Elasmobranch embryos resemble those of the Sturgeon 

 (Acipenser) and the Amphibians in the possession of a spatula- 

 like cephalic expansion: but so far as I am aware a ventral 

 flexure in the medullary plates of the head has not been ob- 

 served in other groups. 



The medullary canal in Elasmobranchs is formed precisely on 

 the type so well recognised for all groups of vertebrates with 

 the exception of the Osseous Fishes. The only feature in any 

 respect peculiar to these fishes is the closing of their medullary 

 canal first commencing behind, and then at a second point in the 



1 Vide Preliminary Account, etc. Q. Jl. Micros, Science, Oct. 1874, PI. 14, 8 a. 

 [This Edition, No. V. PI. 3, 8a.] This and the other section from the same embryo 

 (stage F) may be referred to. I have not thought it worth while repeating them here. 



