THE SPINAL NERVES. 379 



as two club-shaped masses of cells, which have grown outwards 

 and downwards from the extreme dorsal summit of the neural 

 canal and in contact with its walls. The rudiments of the two 

 sides meet at their point of origin at the dorsal median line, 

 and are dorsally perfectly continuous with the walls of the 

 canal. 



It is a remarkable fact that rudiments of posterior roots 

 are to be seen in every section. This may be interpreted as 

 meaning that the rudiments are in very close contact with each 

 other, but more probably means, as I hope to shew in the sequel, 

 that there arises from the spinal cord a continuous outgrowth 

 from which discontinuous processes (the rudiments of posterior 

 roots) grow out. 



After their first formation these rudiments grow rapidly 

 ventral wards in close contact with the spinal cord (vide PI. 14, 

 fig. i, and PI. n, figs. 6 and 7), but soon meet with and become 

 partially enclosed in the mesoblastic tissue (PL 1 1, fig. 7). The 

 similarity of the mesoblast and nerve-tissue in Scyllium and 

 Pristiurus embryos hardened in picric or chromic acid, render 

 the nerves in these genera, at the stage when they first become 

 enveloped in mesoblast, difficult objects to observe ; but no 

 similar difficulty is encountered in the case of Torpedo embryos. 



While the rudiments of the posterior roots are still quite 

 short, those of the anterior roots make their first appearance. 

 Each of these (PI. 14, fig. 4 a. r.) arises as a very small but dis- 

 tinct conical outgrowth from a ventral corner of the spinal cord. 

 From the very first the rudiments of the anterior roots have an 

 indistinct form of peripheral termination and somewhat fibrous 

 appearance, while the protoplasm of which they are composed 

 becomes attenuated towards its end. The points of origin of 

 the anterior roots from the spinal cord are separated by con- 

 siderable intervals. In this fact, and also in the fact of the 

 nerves of the two sides never being united with each other in 

 the median line, the anterior roots exhibit a marked contrast to 

 the posterior. There are thus constituted, before the close of 

 stage I, the rudiments of both the anterior and posterior roots of 

 the spinal nerves. The rudiments of both of these take their 

 origin from the involuted epiblast of the neural canal, and the 

 two roots of each spinal nerve are at first quite unconnected 



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