80 PROFESSOR A. MILNES MARSHALL. 
of the fifth.’ I propose to consider it more fully in a later 
section of this paper. 
The condition of the third nerve and its branches at stage 1 is 
shown in the figs. 10 to 12 and 14, representing longitudinal 
and vertical sections at various depths of the same embryo. In | 
fig. 10, the most superficial of these sections, the ganglion 
(c. g.), is shown wedged in between the first and second head 
cavities; the figure showing, in addition, the roots of the two 
branches (111 @ and 111 4), already described, and also the posterior 
half of the communicating branch (v. d.) between the fifth and 
third nerves. In fig. 11, taken at a deeper level, the anterior 
end of this connecting branch is shown. Fig. 14, at a still 
deeper level, shows the end of the third nerve just before reach- 
ing the ganglion; while, finally, fig. 14, which is the deepest 
section of the series, not far from the midline, shows the greater 
part of the length of the nerve, with the triangular ganglion at 
its root of origin from the mid brain. 
Towards the end of stage ut, besides this ganglionic root 
of origin shown in fig. 14, two or three small additional roots 
are developed ; these are very slender, are situated in front of the 
main ganglionic root, and differ markedly from this root by 
having no ganglion cells. Towards the end of stage 1, then, the 
third nerve arises by one large ganglionic root and two or three 
small, slender, non-ganglionic roots placed in front of the main and 
original root. In stages Mm, N, and o, these small anteriorly 
_ gituated roots become much more evident and also increase in 
number; they are shown at stage o in fig. 40 (ar). The 
morphological significance of these additional roots is discussed 
later on. 
The condition of the nerve between stages L and M is shown 
in the series of horizontal sections represented in figs. 22 to 
24. The section shown in fig. 22 is a curiously lucky one, 
imasmuch as it includes the whole length of the third nerves 
(am) on both sides from their origins from the mid brain to the 
ganglia (e.g.) between the first and second head cavities. The 
roots of the two nerves are seen to arise very close together 
from the ventral surface of the mid brain ; each nerve runs for 
a short distance almost directly backwards, then turns some- 
what outwards, and runs to the interval between the first and 
second head cavities (1 and 2), where it expands into the gan- 
glionic swelling (c.g.). Fig. 23, taken at a rather more dorsal 
level, shows the root of the third nerve on the right-hand side 
and the ganglia (c.g.) on both sides, while fig. 24, at a still 
more dorsal level, shows the communicating branch (v. d.) 
between the ganglia of the third and fifth (v.) nerve. 
' Op. cit., pp. 197, 198, and plate xiv, figs. 9 6 and 16 3. 
