92 PROFESSOR A. MILNES MARSHALL. 
A comparison of these figures with figs. 37 and 38 shows 
that the sixth nerve arises on either side a short distance from 
the mid ventral line of the hind brain by a number of roots ; the 
most anterior of these roots is situated about the level, or imme- 
diately behind the level, of the roots of the seventh nerve, and 
the most posterior of the roots is in front of the middle of the 
_ auditory vesicle. These roots differ totally from the root of the 
seventh nerve—(a) in being multiple instead of single, (b) in being 
devoid of ganglion cells. In these features, as well as in their 
point of origin from the brain, as shown in figs. 29 and 30, they 
bear a very close resemblance indeed to the anterior roots of a 
spinal nerve. Indeed, these two figures seem to me to leave no 
doubt that we must regard the siath nerve as having the same 
relation to the seventh that the anterior root of a spinal nerve 
has to its posterior root. 
In my paper on the cranial nerves of the chick I have already 
expressed the same opinion, and arrived at the same conclusion, 
as to the morphological value of the sixth nerve. 
The question of the existence or non-existence of anterior roots 
to the cranial nerves, interesting enough before, acquires additional 
importance from the arguments based by Mr. Balfour on the 
supposed non-existence of such roots. In Amphioxus all the 
nerves possess single roots, which roots correspond, as Balfour 
has conclusively shown, to the dorsal or posterior roots of other 
Vertebrates. Assuming that this is the primitive condition, of 
which strong evidence is afforded by the actual development of 
the spinal nerves, Balfour, failing to find anterior roots to the 
cranial nerves, argues that the “ retention of the primitive con- 
dition in the brain implies that this became diffentiated from 
the remainder of the nervous system at a very early period before 
the acquirement of anterior nerve roots, and that these eventually 
became developed only in the case of spinal nerves, and not in 
the case of the already highly modified cranial nerves.””1 
It will be seen that this argument amounts to little more than 
an explanation of the supposed absence of anterior roots to the 
cranial nerves, which explanation becomes, of course, unnecessary 
if even a single undoubted anterior root is discovered in a cranial 
nerve. There is no @ priori improbability that such anterior 
roots should exist; on the contrary, it is, to my mind, in the 
highest degree improbable that the cranial nerves should in sueh 
a point have preserved a more primitive arrangement than the 
far less complex spinal nerves. ‘Therefore, when we find a nerve 
in embryos of stage L having the characteristic position, struc- 
ture, and appearance of an anterior spinal root, and bearing the 
same relations to the seventh nerve that an anterior spinal root 
1 Op. cit., p. 193. 
