A474. PROF. MILNES MARSHALL AND W. B. SPENCE, 
from which it arises is of importance, as showing that the fourth 
nerve comes under the category of segmental nerves ;' and inas- 
much as there is no room as regards visceral arches and clefts for 
a segmental nerve between the third and fifth the fourth would 
probably be rightly viewed as a separated branch of the third— 
the only other nerve arising from the mid brain. 
On the other hand, certain other facts in connection with the 
fourth nerve cannot be explained so easily. Thus, segmental 
nerves not only arise from the neural crest ; they also arise early, 
are from the first large, and have ganglia at or near their roots. 
Now, so far as our observations go, the fourth nerve, though it 
may possibly arise from the neural crest, would not appear to arise 
early, and certainly is not at first a large nerve, being of much 
greater size at stage o-p (fig. 15) than at stage n (fig. 11) ; whilst 
at stage L, in the single specimen in whick it has been detected, 
it is a nerve of extreme slenderness; moreover, at no part of its 
length have ganglion cells been found—an important point of 
difference from segmental nerves. 
Another curious feature concerning the fourth nerve is that, 
in our embryos, it appears almost constantly to divide close to 
its root into two or even more branches, as is well shown in fig. 
11, 1v. These again subdivide near their terminations (figs. 
11 and 15), but a// the branches, whether primary or secondary, 
are distributed to the superior oblique muscles, with the possible 
exception of a few of the smaller ones, which appear to join the 
ophthalmic branch of the fifth. We would suggest that this 
branching may possibly be an indication of the fourth nerve 
having previously had a more extended distribution than its 
present very limited one. 
On the whole, our observations lead us to believe that the 
fourth nerve is to be regarded as a separated part of that seg- 
mental nerve of which the third nerve forms the main portion. 
A further suggestion concerning the fourth nerve will be made 
after the seventh nerve has been considered. 
The Fifth (Trigeminal) Nerve.— We propose to consider sepa- 
rately the roots and the branches. 
a. The roots of origin.—The earliest stage in the development 
of the fifth nerve that we propose to treat of in the present paper 
is that represented in fig. 1, taken from a transverse section 
through the hind brain of an embryo at a stage intermediate 
between 1 and kK, the plane of section passing on the left side 
through the roots of the trigeminal (v). 
As shown in the figure, the roof of the hind brain is very 
1 Marshall, ‘“ Morphology of Olf. Organ,” p. 318, ‘Quart. Journ. 
Mier. Sci.,’ July, 1879. 
