160 THE COMMISSURE OF THE POSTERIOR ROOTS. 



grows rapidly duriDg the succeeding stages, and soon forms 

 an elongated cellular structure with a wide attachment to the 



o 



spinal cord (PL xiii. fig. 5). At first it passes obliquely and 

 nearly horizontally outwards, but, before reaching the muscle- 

 plate of its side, takes a bend downwards (PL xiii. fig. 7). 



I have not definitely made out when the anterior and pos- 

 terior roots unite, but this may easily be seen to take place 

 before the close of stage K (PL xi. fig. 18). 



One feature of some interest with reference to the anterior 

 roots, is the fact that they arise not vertically below, but alter- 

 nately with the dorsal roots, a condition which persists in the 

 adult. 



Although I have made some efforts to determine the even- 

 tual fate of the commissure uniting the dorsal roots, these have 

 not hitherto been crowned with success. It grows thinner and 

 thinner, becoming at the same time composed of fibrous pro- 

 toplasm with imbedded nuclei (PL xiii. fig. 8 and 9). By stage 

 M it is so small as to be quite indistinguishable in transverse 

 sections ; and I have failed in stage P to recognize it at all. I can 

 only conclude that it gradually atrophies, and finally vanishes 

 without leaving a trace. Both its appearance and history are 

 very remarkable, and deserve the careful attention of future 

 investigators. 



There can be little doubt that it is some sort of remnant of 

 an ancestral structure in the nervous system ; and it would 

 appear to indicate that the central nervous system must origi- 

 nally have been formed of a median and two lateral strands. 

 At the same time I very much doubt whether it can be 

 brought into relation with the three rows of ganglion-cells 

 (a median and two lateral) which are so frequently present 

 on the ventral side of annelidan nerve-cords. 



My results may he summarised as follows: — Along the 

 extreme dorsal summit of the spinal cord there arises on each 

 side a continuous outgrowth. From each outgrowth processes 

 corresponding in number to the muscle-plates grow downwards. 

 These are the rudiments of the posterior nerve-roots. The 

 outgrowths, though at first attached to the spinal cord through- 

 out their whole length, soon cease to be so, and remain in 

 connection with it at certain points only, which form the 



