482 PROF, MILNES MARSHALL AND W. B, 3PENCER, 
secondary root (vi 3). This latter is now situated still nearer 
to the ventral surface than at its first appearance, the distance 
hetween the two roots being considerably greater than at the 
earlier stages, as is evident from a comparison of fig. 9 with 
fig. 6. The two roots also differ histologically, the dorsal or 
primary root consisting almost entirely of elongated fusiform 
cells, whilst the ventral root (vir (3) is mainly composed of 
spherical ganglion cells. 
This ventral root, at stage o, has, as shown in the figure, two 
distinct attachments to the brain, one just below the other. It 
is shown in longitudinal and vertical section at stage N in 
_ fig. 11, where the brain presents opposite to its point of origin 
an external bulging precisely similar to that opposite to the 
secondary root of the fifth (v 3). The dorsal or primary root 
(vir a) is shown at the same stage in fig. 12. 
In fig. 14 the two roots of the seventh are seen in longitu- 
dinal and vertical section, at an age intermediate between stages 
oandp. The dorsal root (vir a) arises very far up the sides 
of the brain, in fact, as in the earlier stages, from the junction 
of side and roof; itis of considerable length, is widely separated 
from the secondary root, and still consists mainly of fusiform 
cells ; the secondary or ventral root, which is overlaid and almost 
concealed by the primary root, is only seen in part, its most 
anterior portion alone being visible. 
The dorsal or primary root is also well shown at the same 
stage in fig. 15. 
B. Comparison of the embryonic roots of the fifth and seventh 
nerves with those of the adult.—It will be convenient here to 
briefly summarise our results concerning the roots of origin of 
the fifth and seventh nerves, and to trace their changes up to 
the adult form. 
About the close of stage 1 the fifth nerve (fig. 1) still retains 
its primary attachment (va) to the brain, and is on the point 
of acquiring its secondary one (v 8): owing to the growth of 
the roof of the brain the two primary roots, which were at 
first continuous across the top of the brain, are now widely 
separate. The seventh nerve (fig. 2) arises by its primary root 
from the dorsal summit of the brain, whose roof at this point 
has not yet commenced its rapid growth, so that the nerves of 
the two sides are still directly continuous with one another ; 
there is as yet no trace of the secondary root of the seventh. 
At stage x the fifth nerve (fig. 4) has completely lost its 
primary root and is now attached to the brain by the secondary 
root alone (v (3); a slight trace of the former-is still present as 
a small dorsal projection on the nerve just beyond the root of 
