DEVELOPMENT OF THE ELASMOBRANCH FISHES. 93 



(2) That the following parts are respectively formed by the 

 vertebral and lateral plates : 



(a) Vertebral plate. From the splanchnic layer of this, or 

 from cells which appear close to and continuous with it, the 

 skeleton, and connective tissue of the upper part of the body, 

 are derived. 



The remainder of the plate, consisting of a splanchnic and 

 somatic layer, is entirely converted into the muscles of the trunk, 

 all of which are derived from it. 



(b} Between the vertebral plate and the lateral plate is a 

 mass of cells where, as I mentioned above, the mesoblast of the 

 somatopleure and splanchnopleure fuse together. This mass of 

 cells is the equivalent of the intermediate cell mass of Birds 

 (vide Waldeyer, Eierstock und Ei\ 



From it are derived the Wolffian bodies and duct, the 

 oviduct, the ovaries and the testis, and the connective tissue of 

 the parts adjoining these. 



(c) The lateral plate. From the somatic layer of this is 

 derived the connective tissue of the ventral half of the body ; the 

 mesoblast of the limbs, including probably the muscles, and 

 certainly the skeleton. From its splanchnic layer are derived 

 the muscles and connective tissue of the alimentary canal. 



(3) The spinal nerves are developed independently of the 

 protovertebrae, so that the protovertebrae of the Elasmobranchii 

 do not appear to be of such a complicated structure as the proto- 

 vertebrse of Birds. 



Tlic Digestive Canal. 



I do not intend to enter into the whole history of the di- 

 gestive canal, but to confine myself to one or two points of 

 interest connected with it. These fall under two heads : 



(1) The history of the portion of the digestive canal be- 

 tween the anus and the end of the tail where the digestive canal 

 opens into the neural canal. 



(2) Certain less well-known organs derived from the di- 

 gestive canal. 



