DEVELOPMENT OF ELASMOBRANCH FISHES. 83 



In the succeeding stages that part of the epiblast, which will 

 form the spinal cord, gradually becomes two or three cells deep. 

 This change is effected by a decrease in the length of the cells 

 as compared with the thickness of the layer. In the earlier 

 stages the cells are wedge-shaped with an alternate arrange- 

 ment, so that a decrement in the length of the cells at once 

 causes the epiblast to be composed of two rows of interlocking 

 cells. 



The lateral parts of the epiblast which form the epidermis of 

 the embryo are modified in quite a different manner to the 

 nervous parts of the layer, becoming very much diminished in 

 thickness and composed of a single row of flattened cells, 

 PL IX. fig. 3. 



Till the end of stage F, the epiblast cells and indeed all the 

 cells of the blastoderm retain their yolk spherules, but the epi- 

 blast begins to lose them and consequently to become transpa- 

 rent in staofe G. 



Medullary Groove. 



During stage B the medullary groove is shallow posteriorly, 

 deeper in the middle part, and flattened out again at the 

 extreme anterior end of the embryo. PL v. fig. 10, ah c. 



A similar condition obtains in the stage between B and C, 

 but the canal has now in part become deeper. Anteriorly no 

 trace of it is to be seen. In stage C it exhibits the same general 

 features (Plate ix. fig. 2 a 2 6 2 c). 



By stage D we find important modifications of the canal. 



It is still shallow behind and deep in the dorsal region, 

 Plate IX. fig. Sd Se 3/; but the anterior flattened area in the 

 last stage has grown into a round flat plate which may be called 

 the cephalic plate, Plate VI. D and Plate ix. fig. Sa Sb 3 c. 

 This plate becomes converted into the brain. Its size and 

 form give it a peculiar appearance, but the most remarkable 

 feature about it is the ventral curvature of its edges. Its edges 

 do not, as might be expected, bend dorsalwards towards each 

 other, but become sharply bent in a ventral direction. This 

 feature is for the first time apparent at this stage, but becomes 

 more conspicuous during the succeeding ones, and attains its 



G— 2 



