BRANOHIAL SENSE ORGANS IN ICHTHYOPSIDA. 123 
somewhat obliquely to the ciliary, and forms its so-called radix 
longa (fig. 51, ¢. 0.). 
Although I have no observations to record as to the develop- 
ment of the third or motor-oculi nerve, still Marshall’s opinions 
on the nature of the nerve must be discussed, and as his views 
are inconsistent with the other facts as recorded in this paper, 
I shall state what seem to be urgent reasons for modifying 
them. 
Marshall has advanced the suggestion that the third and 
fourth nerves together make up a segmental nerve. He says, 
«There is very strong reason for thinking that, in the chick at 
any rate, the third nerve develops, like the hinder cranial 
nerves and the posterior roots of spinal nerves, as an outgrowth 
from the neural crest on the top of the midbrain.” Since the 
third nerve later on arises from the base of the midbrain, 
“very near the mid-ventral line,” he infers that the nerve must 
shift downwards, and to an extent unequalled by any other 
nerve. 
Now, leaving aside the fact that the shifting in the case of 
the third nerve, if it does take place, occurs, by Marshall’s ad- 
mission, to a greater extent than in the case of the other cra- 
nial nerves, a point which is surely of some importance, there 
are other objections which cannot, I think, be ignored. Mar- 
shall’s views have also been contested by Van Wijhe, for whose 
reasons the reader is referred to the oft-quoted work on the 
nerves of the Elasmobranchii. 
In any discussion as to the nature of the third nerve the 
morphology of the head cavities is bound to have an important 
place. The second or mandibular head cavity undoubtedly 
gives rise to the superior oblique muscle (fig. 12, h. c..). On 
this point I can fully confirm Van Wijhe. 
This fact alone ought to dispose of the fourth nerve, which 
Marshall considers as part of the nerve of the second seg- 
ment, that is, as part of the third nerve. The mandibular 
head cavity arises from the mesoblast plate of the mandibular 
1 Marshall, “ Segmental Value of Cranial Nerves,” ‘Journ. of Anat. and 
Physiol.’ p. 35, 1882. 
