90 PROFESSOR A. MILNES MARSHALL, 
correspond to the buccal branch of the maxillary nerve, but its 
independent origin from the main stem of the seventh seems to 
disprove this. 
The very remarkable communicating branch between the 
Gasserian and ciliary ganglia still remains for consideration ; 
concerning it we know—(a) that it is developed very early, (b) that 
it is a direct connection between the main ganglia of the third 
and fifth nerves. I have no direct observations on its develop- 
ment, but am inclined to think it may be the remains of the 
commissure which (at any rate in the Chick,’ and probably, also, 
in Scylliwm) connects together, at an early stage, the rudiments 
of the third and fifth nerves. This suggestion renders it neces- 
sary for me to abandon the explanation | have previously given? 
of the ophthalmic branches of the seventh and fifth nerves as 
being possibly persistent rudiments of this commissure, as it is 
obvious that the ophthalmic branch (v a.) and the communicat- 
ing branch (v d.) could not both be derived from this com- 
missure. 
Concerning the anterior branch (111 a.) of the third nerve I feel 
in great doubt. It seems possible that it may be an ophthalmic 
branch of the third nerve, equivalent to the ophthalmic branches 
of the fifth or seventh, and its course and distribution certainly 
favour such a view. On the other hand, until the development 
of the fourth nerve has been satisfactorily determined, I think 
any attempt to determine the homologies of this branch of the 
third would be premature. 
The Development of the Sixth (Abducens) Nerve.—Plate 
VI, fig. 38, represents a longitudinal vertical section through 
the head of a Scy//ium embryo at stage o, the section being taken 
a short distance to one side of the median plane; it shows the 
three main divisions of the brain, the pineal gland (piz.) and 
infundibulum, also the lateral expansion (ypzd.) of the pituitary 
involution from the mouth. <A portion of the third nerve (111) is, 
at a short distance from its origin, included in the section, which 
also shows the investing mass (¢.v.) lying at the side of the 
notochord. Springing from the base of the third brain by a 
large number of slender roots and then running forward and 
losing itself in the investing mass is a nerve (v1). In the next 
section this nerve can be seen to come into contact with the end 
of a muscle (7. ¢., fig. 37), which muscle can be traced through 
the more superficial sections in figs. 36, 35, 34, 33, 32, and 
ultimately, in sections not figured here, can be traced into con- 
tact with the eyeball, and can be recognised definitely as the 
1 Vide self, loc. cit. 
2 Loc. cit., p. 30. 
