DEVELOPMENT OF THE ELASMOBRANCH FISHES. 91 



the outermost layer of this mass of cells, which was the primi- 

 tive 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 <?), this outer portion of the splanch- 

 nic layer is completely separated from the skeleton-forming 

 cells, and at the same time, having united below as well as 

 above with the outer (somatic) layer of the two layers of which 

 the protovertebras are formed, the two together form an inde- 

 pendent mass (fig. 12, in p], similar in appearance and in every 

 way homologous with the muscle-plate of Birds. 



On the inner side of this, which we may now call the muscle- 

 plate, is seen the bundle of earlier-developed muscles (fig. 12, 

 mp'} which I spoke of before. 



The section represented in fig. 12 is from a very considerably 

 later embryo than that represented in fig. 11, so that the skele- 

 ton-forming cells, few in number in the earlier section, have 

 become very numerous in the later one, and have grown up 

 above the neural canal, and also below the notochord, between 

 the digestive canal and the aorta. They have, moreover, 

 changed their character ; they were round before, now they 

 have become stellate. As to their further history, it need only 

 be said that the layer of them immediately around the noto- 

 chord and neural canal forms the cartilaginous centra and arches 

 of the vertebrae, and that the remaining portion of them, which 

 becomes much more insignificant in size as compared with the 

 muscles, forms the connective tissue of the skeleton and of the 

 parts around and between the muscles. 



A muscle-plate itself is at this stage (shewn in fig. 12) com- 

 posed of an inner and an outer layer of columnar cells (splanchnic 

 and somatic) united at the upper and lower ends of the plate, 

 and on the inner of the two lies the more developed mass of 

 muscles before spoken of (mp'}. 



Each of these plates now grows both upwards and down- 

 wards ; and at the same time connective-tissue cells appear 

 between the plates and epidermis ; but from where they come 

 I do not know for certain ; very probably they are derived from 

 the somatic layer of the muscle-plate. 



While the muscle- plates continue to grow both upwards and 



