ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 345 



These are formed by the lateral plates of mesoblast, which 

 I said were equivalent at once to the vertebral and lateral 

 plates in the Bird, becoming split by transverse divisions into 

 cubical masses. 



At the time when this occurs, and, indeed, up till a con- 

 siderably later period, the mesoblast is not split into somato- 

 pleure and splanchnopleure, and it is not divided into 

 vertebral and lateral plates. The transverse lines of division 

 of the protovertcbroe do not, however, extend to the outer 

 edge of the undivided lateral plates. 



The differences between this mode of formation of the 

 protovertebrse and that occurring in Birds are too obvious 

 to require pointing out. I will speak of them more fully 

 when I have given the whole history of the protovertebrse 

 of the Dog-fish. 



I will only now say that I have had in the early stages to 

 investigate the formation of the protovertebrge entirely by 

 means of sections, the objects being too opaque to be other- 

 wise studied. 



The next change of any importance is the commencement 

 of the formation of the head. The region of the head first 

 becomes distinguishable by the flattening out of the germ at 

 its front end. 



The flattened-out portion of the germ grows rapidly, and 

 forms a spatula-like termination to the embryo (PI. XIII, 

 fig. 8). 



In the region of the head the medullary groove is at first 

 totally absent {vide section, PI. XIV, fig. 8 a). 



Indeed, as can be seen from fig. 8 6, the laminse dorsales, so 

 far from bending up at this stage, actually bend down in the 

 opposite direction. 



I am at present quite unable even to form a guess what 

 this peculiar feature of the brain means. It, no doubt, has 

 some meaning in reference to the vertebrate ancestry if we 

 could only discover it. The peculiar spatula- like flattened 

 condition of the head is also [vide loc. ant. cit.) appa- 

 rently found in the Sturgeons ; it must therefore almost 

 undoubtedly be looked upon as not merely an accidental 

 peculiarity. 



While these changes have been taking place in the head 

 not less important changes have occurred in the remainder 

 of the body. In the first place the two caudal lobes have 

 increased in size, and have become, as it were, pushed in 

 together, leaving a groove betAveen them (fig. 8, t s). They 

 are very conspicuous objects, and, together with the spatula- 

 like head, give the whole embryo an almost comical appear- 



