DEVELOPMENT OF THE ELASMOBRANCH FISHES. 85 



a share in its formation in the Dog-fish. I have not as yet 

 made out for certain the share which is taken by these freshly 

 formed cells of the yolk in the formation of any other organ. 



By the completion of its lower wall in the way described, 

 the throat early becomes a closed tube, its closing taking place 

 before any other important changes are visible in the embryo 

 from surface views. 



A considerable increase in length is attained before other 

 changes than an increase in depth of the medullary groove and 

 a more complete folding off of the embryo from the blastoderm 

 take place. The first important change is the formation of the 

 protovertebrae. 



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 proto- 

 vertebrae do not, however, extend to the outer edge of the 

 undivided lateral plates. 



The differences between this mode of formation of the pro- 

 tovertebrae 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 protovertebrae of the Dog-fish. 



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

 to investigate the formation of the protovertebrae 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 (PL 3, fig. 8). 



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

 totally absent (vide section, PI. 3, fig. 8 a). 



Indeed, as can be seen from fig. 8 b, the laminae dorsales, so 



