1/8 DEVELOPMENT OF THE SPINAL NERVES 



In a stage somewhat subsequent to this I have succeeded in 

 making longitudinal sections, which exhibit these junctions with 

 a clearness which leaves nothing to be desired. 



It is there effected (PI. 23, fig. L) in each case by a proto- 

 plasmic commissure with imbedded nuclei 1 . Near its dorsal 

 extremity each posterior root dilates, and from the dilated por- 

 tion is given off on each side the commissure uniting it with the 

 adjoining roots. 



Considering the clearness of this formation in this embryo, 

 as well as in the embryo belonging to the stage under descrip- 

 tion, there cannot be much doubt that at the first formation 

 of the posterior rudiments a continuous outgrowth arises from 

 the spinal cord, and that only at a later period do the junctions 

 of the roots with the cord become separated and distinct for 

 each nerve. 



I now return to the more complete series of Pristiurus- 

 embryos, the development of whose spinal nerves I have been 

 able to observe. 



Second Stage of the Spinal Nerves in Pristiums. 



In the youngest of these (PI. 22, fig. E) the notochord has 

 undergone but very slight changes, but the segmental duct has 

 made its appearance, and is as much developed as in the Torpedo- 

 embryo from which fig. D b was taken. 



(The embryo from which fig. E a was derived had three 

 visceral clefts.) 



There have not as yet appeared any connective-tissue cells 

 dorsal to the top of the muscle-plates, so that the posterior 

 nerve-rudiments are still quite free and distinct. 



The cells composing them are smaller than the cells of the 

 neural canal ; they are round and nucleated ; and, indeed, in 

 their histological constitution the nerve-rudiments exhibit no 

 important deviations from the previous stage, and they have 

 hardly increased in size. In their mode of attachment to the 

 neural tube an important change has, however, already com- 

 menced to be visible. 



In the previous stage the two nerve-rudiments met above the 



1 This commissure is not satisfactorily represented in the figure. Vide Explana- 

 tion of Plate 2. 



