IN ELASMOBRANCH FISHES. 177 



anterior root are not less important than those in the posterior. 

 The anterior root now forms a very conspicuous cellular promi- 

 nence growing out from the ventral corner of the spinal cord 

 (fig. D c, ar). It has a straight course from the spinal cord 

 to the muscle-plate, and there shews a tendency to turn down- 

 wards at an open angle : this, however, is not represented in the 

 specimen figured. The cells of which it is composed each con- 

 tain a large oval nucleus, and are not unlike the cells which 

 form the posterior rudiment. The anterior and posterior nerves 

 are still quite unconnected with each other ; and in those sec- 

 tions in which the anterior root is present the posterior root 

 of the same side is either completely absent or only a small 

 part is to be seen. The cells of the spinal cord exhibit a 

 slight tendency to converge towards the origin of the anterior 

 nerve-root. 



In the spinal cord itself the epithelium of the central canal 

 is commencing to become distinguished from the grey matter, 

 but no trace of the white matter is visible. 



I have succeeded in making longitudinal vertical sections of 

 this stage, which prove that the ends of the posterior roots 

 adjoining the junction with the cord are all connected with each 

 other (PL 22, fig. D d). 



If the figure representing a transverse section of the em- 

 bryo (fig. D c) be examined, or better still the figure of a section 

 of the slightly older 8cy Ilium-embryo (PL 23, fig. H I or 1 1), 

 the posterior root will be seen to end dorsally in a rounded 

 extremity, and the junction with the spinal cord to be effected, 

 not by the extremity of the nerve, but by a part of it at some 

 little distance from this. 



It is from these upper ends of the rudiments beyond the 

 junction with the spinal cord that I believe the commissures to 

 spring which connect together the posterior roots. 



My sections shewing this for the stage under consideration 

 are not quite as satisfactory as is desirable ; nevertheless they 

 are sufficiently good to remove all doubt as to the presence of 

 these commissures. 



A figure of one of these sections is represented (PL 22, fig. 

 D d). In this figure pr points to the posterior roots and x to 

 the commissures uniting them. 



B. 12 



