92 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



downwards, the cells of which they are composed commence to 

 become elongated and soon acquire an unmistakably muscular 

 character (PI. 4, fig. 13, mp). 



Before this has occurred the inner mass of muscles has 

 also undergone further development and become a large and 

 conspicuous band of muscles close to the notochord (fig. 

 13, mp'}. 



At the same time that the muscle-plates acquire the true 

 histological character of muscle, septa of connective tissue grow 

 in and divide them into a number of distinct segments 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 in- 

 vestigated) are derived from the muscle-plates which grow up- 

 wards 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 attach- 

 ment to the neural tube being still at its posterior (dorsal) 

 surface, near which they first appeared. Later still 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 protovertebras are 



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

 horizontally into somatopleure and splanchnopleure the verte- 

 bral and lateral plates are one, and the splitting extends to the 

 very top of the vertebral or muscle-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. 



