IN ELASMOBRANCH FISHES. 183 



changes both in the nerves themselves and in the connective- 

 tissue around them, which becomes less compact and its cells 

 stellate, the difficulty of distinguishing the nerves from the 

 surrounding cells vanishes ; and the difficulties of investigation 

 in the later stages are confined to the modes of attachment of 

 the nerves to the neural canal, and the histological changes 

 which take place in the rudiments themselves. 



The stage may be considered to commence at the period 

 when the external gills first make their appearance as small 

 buds from the walls of the visceral clefts. Already, in the 

 earliest rudiments of the posterior root of this period now 

 figured, a number of distinct parts are visible (PL 23, fig. H i). 



Surrounding nearly the whole structure there is present a 

 delicate investment similar to that which I mentioned as sur- 

 rounding the neural canal and other organs ; it is quite struc- 

 tureless, but becomes coloured with all staining reagents. I 

 must again leave open the question whether it is to be looked 

 upon as a layer of coagulated protoplasm or as a more definite 

 structure. This investment completely surrounds the proxi- 

 mal portion of the posterior root, but vanishes near its distal 

 extremity. 



The nerve-rudiment itself may be divided into three distinct 

 portions : (i) the 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 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 remark- 

 ably enough, not by its extremity, but by its side, to the spinal 

 cord. The dorsal extremities of the posterior nerves are there- 

 fore free ; as was before mentioned, they probably serve as the 

 starting-point of the longitudinal commissures between the 

 posterior roots. 



The spinal cord at this stage is still made up of fairly uni- 

 form cells, which do not differ in any important particulars from 

 the cells which composed it during the last stage. The outer 

 portion of the most peripheral layer of cells has already begun to 

 be converted into the white matter. 



The delicate investment spoken of before still surrounds the 



