OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM, 481] 
that this curious shifting of the nerve roots, though clearly a 
change of a secondary nature, must yet have been acquired very 
early by Vertebrates. 
The later stages of development of the roots of the seventh 
also present points of great interest. Fig. 6 represents a trans- 
verse section through the roots of the seventh nerve of the same 
embryo of stage n, of which fig. 5 shows the rootsof v. The 
seventh nerve is seen to rise on either side by two roots, one 
(vit a) from the top of the sides of the brain at the junction of 
the thickened side with the thin roof, while the other (vm (3) 
arises about half way down the sides of the brain. Between the 
two roots the nerve is in contact with the brain, but apparently | 
not connected with it. We have traced the intermediate steps 
between figs. 3 and 6, and find that the upper root (vit a) of 
fig. 6 is the original dorsal or primary root, and the lower one 
(vit (3) the secondary root of fig. 3. In other words, there is 
an important difference between the fifth and the seventh, 
inasmuch as in the former the primary root is lost and the 
secondary alone retained, whilst im the latter both primary and 
secondary roots are retained up to stage N, and, indeed, as we 
shall see immediately, throughout life. The difference between 
the roots of the fifth and seventh nerves just noticed, does not 
occur in the case of the chick, in which the primary root of the 
seventh is lost as completely as is that of the fifth in Hlasmo- 
branchs.* : 
This shifting of the roots of origin and acquiring of a 
secondary connection with the sides of the brain is not confined 
tothe cranial nerves. It has already been shown to occur in the 
posterior roots of the spinal nerves of the chick,? and occurs also 
in the posterior roots of the spinal nerves of Elasmobranchs, 
It is a point of much interest to note that the seventh nerve, in 
the retention of its primary as well as its secondary root, 7s not 
only more primitive than the fifth, but more primitive even than 
the spinal nerves. 
The condition of the roots of the seventh at stage o is shown 
in the left hand side of fig. 9, representing half of a transverse 
section through the hind brain and roots of origin of this nerve. 
The two roots, the primary (vi a) and the secondary (vm (3), 
are even more distinct than at the earlier stages. The primary 
root (vit a) arises as before from the top of the thickened side 
of the hind brain just before its junction with the thin roof; 
from this origin the root runs downward, alongside of, and 
closely applied to the brain, but unconnected with it, to join the 
1 Marshall, ‘ Quart, Journ. Micr. Sci.,’ Jan., 1878, pp. 24 and 25. 
.” Marshall, “On the Early Stages of Development of the Nerves in 
Birds,” ‘ Journal of Anatomy,’ vol. xi, 1877. 
