158 FIRST FORMATION OF THE SPINAL NERVES. 



early development are sufficiently important to call for atten- 

 tion. 



One feature of the posterior roots at tlieir 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 connecting 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 re- 

 markable results of my researches upon the Elasmobranch 

 nervous system. At stage K it is fairly tliick, though it becomes 

 much thinner at a slightly later period. Its condition during 

 stage K is shown in Plate xi. fig. 18, com. What it has been 

 possible for me to make out of its eventual fate is mentioned 

 subsequently \ 



A second feature of the eaiiiest 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, 

 and I do not feel quite satisfied that I have yet got completely 

 to the bottom of its meaning. The explanation which appears- 

 to me most probable has suggested itself in the course of some 

 observations on the development of the thin roof of the fourth 



1 It is not by any means always possible to detect tliis commissure in trans- 

 verse sections. As I hnve suggested, in connection with a similar commissure 

 counecting the vagus branches, it perhaps easily falls out of the section, and is 

 always so small that the hole left would certainly be invisible. 



