480 PROF, MILNES MARSHALL AND W. B. SPENCER. 
takable, while between them the nerve and brain are quite dis- 
tinct from one another, and separated by an appreciable interval. 
If tig. 3, showing the condition of the root of the seventh 
nerve at stage K, be compared with fig. 1, showing the root of 
the fifth nerve at a rather earlier stage, it will be seen at once 
that there is a very close resemblance between the two; the sole 
point of difference being that in fig. 1, though the nerve still 
retains its primary attachment, the secondary has not yet been 
actually acquired. Balfour’s figures and description, already 
referred to, show that at a still earlier stage the fifth nerve has 
exactly the same appearance and relations which the seventh has 
in fig. 2; and it is mainly on this fact, coupled with the close 
similarity between such specimens as those represented in figs. 1 
and 3, that we rely in support of the explanation we have given 
above of the development of the root of the fifth nerve. 
Inasmuch as figs. 1 and 2 are taken from the same embryo, it 
would seem that the fifth nerve appears before the seventh, and 
is, during the earlier phases of development, just one stage ahead 
of it in development. At a time (fig. 1) when the primary roots 
of the fifth have already become widely separated by growth 
of the brain-roof, and the secondary attachment (v @) is on the 
point of being acquired, the two seventh nerves (fig. 2) are still 
im contact with one another across the top of the unexpanded 
brain-roof; and at stage «x the seventh nerve (fig, 3) is in 
exactly the same condition as the fifth at the end of stage 1 
fig. 1). 
‘Our observations appear, therefore, to prove conclusively that 
as concerns the seventh nerve, while the change of position of 
the dorsal or primary root (vit a) is due solely to rapid growth of 
the roof of the brain, the lower or ventral root (vit 3) is a new 
and purely secondary attachment. 
Whilst these results concerning the roots of the seventh are, 
we believe, new as applied to Elasmobranchs, they are in perfect 
accordance with the account previously given by one of us of the 
development of the seventh nerve in the chick, in which the 
very same series of changes—the separation of the primary 
roots by growth of the brain-roof, and the acquiring of new or 
secondary roots—are shown to occur in a precisely similar man- 
ner.! The close correspondence between these two very different 
types of vertebrates is of much interest, partly as tending to 
confirm the correctness of the account, and partly as showing 
1 Marshall, ‘‘Develop. of Cranial Nerves in Chick,” ‘ Quart. Journ. 
Micr. Sci.,’ Jan., 1878, pp. 34 and 35. 
The prediction there made, that the secondary attachment of the nerves 
in Elasmobranchs would prove on further investigation to be acquired in 
exactly the same manner as in the chick, is now completely verified. 
