DEVELOPMENT OF ELASMOBRANCH FISHES. 157 



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 show 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 ven- 

 tralwards in close contact with the spinal cord (vide PI. xiii. fig. 

 1, and PL X. figs. 6 and 7), but soon meet with and become 

 partially enclosed in the mesoblastic tissue (PI. x. fi^. 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. xiii. 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 some- 

 what 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 considerable 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 with each other. It is scarcely neces- 

 sary 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 



