380 DEVELOPMENT OF ELASMOBRANCH FISHES. 



with each other. It is scarcely necessary to state that the pairs 

 of roots correspond in number with the muscle-plates. 



It is not my intention to enter with any detail into the 

 subsequent changes of the rudiments whose origin has been 

 described, but a few points especially connected with their early 

 development are sufficiently important to call for attention. 



One feature of the posterior roots at their first formation 

 is the fact that they appear as processes of a continuous out- 

 growth of the spinal cord. This state of affairs is not of long 

 continuance, and before the close of stage I each posterior root 

 has a separate junction with the spinal cord. What then be- 

 comes of the originally continuous outgrowth ? It has not been 

 possible for me to trace the fate of this step by step ; but the 

 discovery that at a slightly later period (stage K) there is present 

 a continuous commissure independent of the spinal cord connect- 

 ing the dorsal and central extremities of all the spinal nerves, 

 renders it very probable that the original continuous outgrowth 

 becomes converted into this commissure. Like all the other 

 nervous structures, this commissure is far more easily seen in 

 embryos hardened in a mixture of osmic and chromic acids or 

 osmic acid, than in those hardened in picric acid. Its existence 

 must be regarded as one of the most remarkable results of my 

 researches upon the Elasmobranch nervous system. At stage K 

 it is fairly thick, though it becomes much thinner at a slightly 

 later period. Its condition during stage K is shewn in PI. 12, 

 fig. 1 8, com. What it has been possible for me to make out of its 

 eventual fate is mentioned subsequently 1 . 



A second feature of the earliest condition of the posterior 

 roots is their attachment to the extreme dorsal summit of the 

 spinal cord a point of attachment very different from that 

 which they eventually acquire. Before the commencement of 

 stage K this state of things has become altered ; and the pos- 

 terior roots spring from the spinal cord in the position normal 

 for Vertebrates. 



This apparent migration caused me at first great perplexity, 



1 It is not by any means always possible to detect this commissure in transverse 

 sections. As I have suggested, in connection with a similar commissure connecting 

 the vagus branches, it perhaps easily falls out of the section, and is always so small 

 that the hole left would certainly be invisible. 



