90 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



In any case, it is clear that the condition in the Bird, where 

 the spinal nerves are derived from tissue of the protovertebrae, 

 is not the primitive one. Of this, however, I will speak again 

 when I have concluded my account, of the development of the 

 protovertebrae. 



About the same time that the first rudiments of the nerves 

 appear, the division of the mesoblast of the sides of the body 

 into a vertebral and a lateral portion occurs. This division first 

 appears in the region where the oviduct (M tiller's duct) is formed 

 (PL 4, fig. 11,0V). 



At this part opposite the level of the dorsal aorta the two 

 sheets, viz. the splanchnic and the somatic, unite together, and 

 thus each lateral sheet of mesoblast becomes divided into an 

 upper portion (fig. u, nip), split up by transverse partitions into 

 protovertebrae, and a lower portion not so split, but consisting of 

 an outer layer, the true somatopleure, and an inner layer, the 

 true splanchnopleure. These two divisions of the primitive plate 

 are thus separated by the line at which a fusion between the 

 mesoblast of the somatopleure and splanchnopleure takes place. 

 The mass of cells resulting from the fusion at this point cor- 

 responds with the intermediate cell-mass of Birds (vide Waldeyer, 

 Eierstock tmd Ei). 



At the same time, in the upper of these two sheets (the pro- 

 tovertebrae), the splanchnic layer sends a growth of cells in- 

 wards towards the notochord and the neural canal. This growth 

 is the commencement of the large quantity of mesoblastic 

 tissue around the notochord, which is in part converted into 

 the axial skeleton, and in part into the connective tissue ad- 

 joining this. 



This mass of cells is at first quite continuous with the 

 splanchnic layer of the protovertebrae, and I see no reason 

 for supposing that it is not derived from the growth of the 

 cells of this layer. The ingrowth to form it first appears a 

 little after the formation of the dorsal aorta ; but, as far as 

 I have been able to see, its cells have no connection with the 

 walls of the aorta. 



What I have said as to the development of the skeleton- 

 forming layer will be quite clear from figs. II and \2a; and 

 from these it will also be clear, especially from fig. 1 1 a, that 



