ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 349 



our present knowledge of wandering cells, must not be thrown 

 aside as altogether improbable. 



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

 where the spinal nerves are derived from tissue of the proto- 

 vertebrse, 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 I have found the rudiments of 

 the nerves 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 (Miiller's 

 duct) is formed (PI. XV, fig. 11, ov). 



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. 11, mp), split up by transverse partitions 

 into protovertebrae, and a lower portion not so split, but con- 

 sisting 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 splanchno- 

 pleure takes place. The mass of cells resulting from the 

 fusion at this point corresponds with the intermediate cell- 

 mass of Birds [vide Waldeyer, ^Eierstock und Ei^j. 



At the same time, in the upper of these two sheets, the 

 splanchnic layer sends a growth of cells inwards 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 adjoining 

 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. 11 and 12 a; 

 and from these it will also be clear, especially from fig. 11a, 

 that the outermost layer of this mass of cells, which was the 

 primitive splanchnic layer of the protovertebrae, still retains 

 its epithelial character, and so can easily be distinguished 

 from those cells which will form the skeleton. In the next 

 stage which I have figured (fig. 12 a), this outer portion of the 



