TN ELASMOBRANCH FISHES. l8l 



their observation is involved in the same difficulties with 

 chromic-acid specimens as that of the posterior rudiments. 



There is a further difficulty in observing the anterior roots, 

 which arises from the commencing formation of white matter in 

 the cord. This is present in all the anterior sections of the 

 embryo from which fig. F is taken. When the white matter is 

 formed the cells constituting the junction of the anterior nerve- 

 root with the spinal cord undergo the same changes as the cells 

 which are being converted into the white matter of the cord, and 

 become converted .into nerve-fibres ; these do not stain with 

 haematoxylin, and thus an apparent space is left between the 

 nerve-root and the spinal cord. This space by careful examina- 

 tion may be seen to be filled up with fibres. In osmic acid 

 sections, although even in these the white matter is stained less 

 deeply than the other tissues, it is a matter of comparative ease 

 to observe the junction between the anterior nerve root and 

 the spinal cord. 



I have been successful in preparing satisfactory longitudinal 

 sections of embryos somewhat older than that shewn in fig. F, 

 and they bring to light several important points in reference to 

 the development of the spinal nerves. Three of these sections 

 are represented in PI. 22, figs. G I, G 2, and G 3. 



The sections are approximately horizontal and longitudinal. 

 G i is the most dorsal of the three ; it is not quite horizontal 

 though nearly longitudinal. The section passes exactly through 

 the point of attachment of the posterior roots to the walls of the 

 neural canal. 



The posterior rudiments appear as slight prominences of 

 rounded cells projecting from the wall of the neural canal. 

 From transverse sections the attachment of the nerves to the 

 wall of the neural canal is proved to be very narrow, and from 

 these sections it appears to be of some length in the direction of 

 the long axis of the embryo. A combination of the sections 

 taken in the two directions leads to the conclusion that the nerves 

 at this stage thin out like a wedge before joining the spinal cord. 



The independent junctions of the posterior rudiments with 

 the spinal cord at this stage are very clearly shewn, though the 

 rudiments are probably united with each other just dorsal to 

 their junction with the spinal cord. 



