ON THE DEVELOPMENT OF THE ELASMOBKA NCH FISHES. 85 1 



ments, which subsequently form separate bands of muscle. 

 I will not say more in reference to the development of the 

 muscular system than that the whole of the muscles of the 

 body (apart from the limbs, the origin of whose muscular 

 system I have not yet investigated) are derived from the 

 muscle-plates which grow upwards above the neural canal and 

 downwards to the ventral surface of the body. 



During the time the muscle-plates have been undergoing 

 these changes the nerve masses have also undergone develop- 

 mental changes. 



They become more elongated and fibrous^ their main 

 attachment to the neural tube being still at its posterior 

 (dorsal) surface, near which they first appeared. Later they 

 become applied closely to the sides of the neural tube and 

 send fibres to it below as well as above. Below (ventral to) 

 the neural tube a ganglion appears, forming only a slight 

 swelling, but containing a number of characteristic nerve- 

 cells. The ganglion is apparently formed just below the 

 junction of the anterior and posterior roots, though probably 

 the fibres of the two roots do not mix till below it. 



The main points which deserve notice in the development 

 of the protovertebrae are — 



(1) That at the time when the mesoblast becomes split 

 herizontally into somatopleure and splanchnopleure the 

 vertebral and lateral plates are one, and the splitting extends 

 to the very top of the vertebral plate, so that the future 

 muscle-plates are divided into a splanchnic and somatic 

 layer, the space between which is at first continuous with 

 the pleuro-peritoneal cavity. 



(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 uud 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. 



