ON THE DEVELOPMENT OF THK ELASMOBRANCH FISHES. 347 



vertebrae, ul ^vllich about twenty are present, can now be 

 seen in the fresh specimens. The heart, however, is not yet 

 formed. 



Up to this period, a period at which the embryo becomes 

 very similar in external appearance to any other vertebrate 

 embryo, I have followed in my description a chronological 

 order. I shall now cease to do so, since it would be too long 

 for a preliminary notice of this kind, but shall confine myself 

 to the history of a few organs whose development is either 

 more important or more peculiar than that of the others. 



The Protovertebrce. 



I have thought it worth while to give a short history of the 

 development of the protovertebrse, firstly, because it is very 

 easy to follow this in the Dog-fish, and, secondly, because I 

 believe that the Dog-fish have more nearly retained the primi- 

 tive condition of the protovertebrse 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. XIV, fig. ^ a, v p), but which had 

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

 outer limit of the sheet, but as yet not cutofFby longitudinal 

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

 vertebrse. 



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 subse- 

 quent 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 splanchno- 

 pleure commences (PI. XIV, 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 protovertebrse 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 protovertebrse, because at this time no separa- 

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

 anticipate matters by saying that only the upper portion of the 

 sheet from the level of the top of the digestive i^anal, becomes 

 subsequently the true protovertebr^,; sp that it is clear that 



