88 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



I believe that the Dog-fish have more nearly retained the pri- 

 mitive condition of the protovertebrae than any other vertebrate 

 whose embryology has hitherto been described with sufficient 

 detail. 



I intend to describe, at the same time, the development of 

 the spinal nerves. 



I left each lateral mass of mesoblast in my last stage as 

 a plate which had not yet become split into a somatic and 

 a splanchnic sheet (PI. 3, fig. 8 a, v /), but which had be- 

 come cut by transverse lines (not, indeed, extending to the 

 outer limit of the sheet, but as yet not cut off by longitudi- 

 nal lines of cleavage) into segments, which I called proto- 

 vertebrae. 



This sheet of mesoblast is fairly thick at its proximal (upper) 

 end, but thins off laterally to a sheet two cells deep, and its 

 cells are so arranged as to foreshadow its subsequent splitting 

 into somatic and splanchnic sheets. Its upper (proximal) end 

 is at this stage level with the bottom of the neural canal, but 

 soon begins to grow upwards, and at the same time the splitting 

 into somatopleure and splanchnopleure commences (PI. 3, fig. 10, 

 so and sp). 



The separation between the two sheets is first visible in its 

 uppermost part, and thence extends outwards. By this means 

 each of the protovertebrae becomes divided into two sheets, 

 which are only connected at their upper ends and outside the 

 region of the body. I speak of the whole lateral sheet as being 

 composed of protovertebrae, because at this time no separa- 

 tion into vertebral and lateral plates can be seen ; but I may 

 anticipate matters by saying that only the upper portion of the 

 sheet from the level of the top of the digestive canal, becomes 

 subsequently the true protovertebrae. From this it is clear that 

 the pleuro-peritoneal cavity extends primitively quite up to the 

 top of the protovertebrae ; and that thus a portion of a sheet of 

 mesoblast, at first perfectly continuous with the splanchnic sheet 

 from which is derived the muscular wall of the alimentary canal, 

 is converted into a part of the voluntary muscular system of the 

 body, having no connection whatever with the involuntary mus- 

 cular system of the digestive tract. 



The pleuro-peritoneal cavity is first distinctly formed at a 



