DEVELOPMENT OF ELASMOBRANX'H FISHES. 159 



ventricle. A growth of cells appears to take place in the median 

 dorsal line of the roof of the spinal cord. This growth tends to 

 divaricate the two lateral parts of the cord, which are originall}'- 

 contiguous in the dorsal line, and causes therefore the posterior 

 roots, which at first spring from the dorsal summit, to assume 

 an apparent attachn^ent to the side of the cord at some little 

 distance from the summit. If this is the true explanation of 

 the change of position wdiich takes place, it must be regarded 

 as due rather to peculiar growths in the spinal cord, than 

 to any alteration in the absolute attachment of the nerves. 



By stage K the rudiment of the posterior root has become 

 greatly elongated, and exhibits a division into three distinct 

 portions (PL XIII. fig. 6) : 



(1) A proximal portion, in which is situated the pedicle of 

 attachment to the wall of the neural canal. 



(2) an enlarged portion, which may conveniently from its 

 future fate be called the spinal ganglion. 



(3) a distal portion beyond this. 



The proximal portion presents a fairly uniform diameter, and 

 ends dorsally in a rounded expansion ; it is attached, remarkably 

 enough, not hy its extremity/, but hy its side, to the spinal cord. 

 The dorsal extremities of the posterior roots are therefore free. 

 It seems almost certain that the free dorsal extremities of these 

 roots serve as the starting points for the dorsal commissure 

 before mentioned, which connects the roots together. The 

 attachment of the posterior nerve-root to the spinal cord is, 

 on account of its small size, very difficult to observe. In 

 favourable specimens there may however be seen a distinct 

 cellular prominence froni the spinal cord, which becomes con- 

 tinuous with a small prominence on the lateral border of the 

 nerve-root near its distal extremity. The proximal extremity 

 of the rudiment is composed of cells, which, by their small size 

 and circular form, are easily distinguished from those which 

 form the succeeding or ganglionic portion of the nerve. This 

 succeeding part has a swollen configuration, and is composed 

 o.f large elongated cells with oval nuclei. The remainder of the 

 rudiment forms the commencement of the true nerve. 



The anterior root, which, at the close of stage I, formed a 

 small and inconspicuous pruminencc from tlie spinal cord. 



