184 
ME. E. M. BALFOUE ON THE DEVELOPMENT OP 
The anterior roots have grown considerably in length, but 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 hsematoxylin, and 
thus an apparent space is left between the nerve-root and the spinal cord. This space by 
careful examination 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 shown 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 Plate 17. figs. G 1, G 2, & G 3. 
The sections are approximately horizontal and longitudinal. G 1 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 shown, though the rudiments are probably united with each 
other just dorsal to their junction with the spinal cord. 
The nerves correspond in number with the muscle-plates, and each arises from the spinal 
cord, nearly opposite the middle line of the corresponding muscle-plates (figs. G 1 & G 2). 
Each nerve-rudiment is surrounded by connective-tissue cells, and is separated from 
its neighbours by a considerable interval. 
At its origin each nerve-rudiment lies opposite the median portion of a muscle-plate 
(figs. G 1 & G 2) ; but, owing to the muscle-plate acquiring an oblique direction, at the 
level of the dorsal surface of the notochord it appears in horizontal sections more 
nearly opposite the interval between two muscle-plates (figs. G 2 & G 3). 
In horizontal sections I find masses of cells which make their appearance on a level 
with the ventral surface of the spinal cord. I believe I have in some sections success- 
fully traced these into the spinal cord, and 1 have little doubt that they are the 
