186 
ME. F. M. BALFOUE ON THE DEVELOPMENT OE 
differ in any important particulars from the cells which composed it during the last stage. 
The outer portion of the most peripheral layer of cells has already begun to be converted 
into the white matter. 
The delicate investment spoken of before still surrounds the whole spinal cord, except 
at the points of junction of the cord with the nerve-rudiments. Externally to this in- 
vestment, and separated from it for the most part by a considerable interval, a meso- 
blastic sheath (Plate 17. fig. H i, i ) for the spinal cord is beginning to be formed. 
The attachment of the nerve-rudiments to the spinal cord, on account of its 
smallness, is still very difficult to observe. In many specimens where the nerve is visible 
a small prominence may be seen rising up from the spinal cord at a point corresponding 
to x (Plate 17. fig. H i). It is, however, rare to see this prominence and the nerve con- 
tinuous with each other : as a rule they are separated by a slight space, and frequently 
one of the cells of the mesoblastic investment of the spinal cord is interposed between 
the two. In some especially favourable specimens, similar to the one figured, there can 
be seen a distinct cellular prominence (fig. H i, x) from the spinal cord, which becomes 
continuous with a small prominence on the lateral border of the nerve-rudiment near its 
free extremity. The absence of a junction between the two in a majority of sections 
is only what might be expected, considering how minute the junction is. 
Owing to the presence of the commissure connecting the posterior roots, some part of 
a nerve is present in every section. 
The proximal extremity of the nerve-rudiment itself is composed of cells, which, by 
their smaller size and a more circular form, are easily distinguished from cells forming 
the ganglionic portion of the nerve. 
The ganglionic portion of the nerve, by its externally swollen configuration, is at once 
recognizable in all the sections in which the nerve is complete. The delicate investment 
before mentioned is continuous around it. The cells forming it are larger and more 
elongated than the cells forming the upper portion of the nerve-rudiment : each of 
them possesses a large and distinct nucleus. 
The remainder of the nerve-rudiment forms the commencement of the true nerve. It 
can in this stage be traced only for a very small distance, and gradually fades away, in 
such a manner that its absolute termination is very difficult to observe. 
The connective-tissue cells which surround the nerve-rudiment are far looser than 
in the last stage, and are commencing to throw out processes and become branched. 
The anterior root-nerve has grown very considerable since the last stage. It projects 
from the same region of the cord as before, but on approaching the muscle-plate takes 
a sudden bend downwards (fig. H n, ar ). 
I have failed to prove that the anterior and posterior roots are at this stage united. 
Fourth Stage.] 
In an embryo but slightly more advanced than the one last described, important steps 
have been made in the development of the nerve-rudiment. The spinal cord itself now 
