470 PROF. MILNES MARSHALL AND W, B. SPENCER, 
Our investigations have been conducted almost exclusively by 
means of sections of hardened embryos of Scy//iwm, and, as on 
former occasions, we have found a mixture of chromic and osmic 
acids superior to any other hardening agent. For the specimens 
from which our best preparations have been made we are indebted 
to the courtesy of the managers of the Southport Aquarium ; our 
best thanks are also due to Mr. A. J. Moss, of Owens College, 
for his gift of a fine specimen of J/ustelus, as well as for valuable 
assistance in connection with the literature of our subject. 
The Third (Oculomotor) Nerve.— We do not propose to deal in 
the present paper with either the olfactory or optic nerves, inas- 
much as the former has been already fully described,! while con- 
cerning the latter we have nothing new to communicate; we 
therefore commence with the third or oculomotor nerve. 
Concerning the development of this nerve we have very little 
to add to the account given in the paper already referred to.? 
At stage L it arises from the base of the mid brain, not far from 
the mid ventral line, by a large posterior ganglionic root and by 
several smaller anterior ones clearly distinguished from the 
former by possessing no ganglion cells. The nerve itself runs 
backwards as a long slender stem to the interval between the 
first and second head cavities (fig. 11 0., 111), where it expands 
into a ganglionic swelling (fig. 11, 0. c.g.) wedged in between 
the tops of the two cavities. From this ganglion the two 
main branches of the third arise; of these the upper one (fig. 
15, m1 4), at a rather later stage, supplies the rectus superior 
and rectus imternus muscles, whilst the lower one (fig. 15, 
1m ¢) runs down behind the rectus inferior, and ends in the 
obliquus inferior muscle (fig. 15,0.2.). Atstage K, at which our 
observations commence, the third nerve has the same point of origin 
and the same relation to the head cavities; it differs from the 
condition described above chiefly in not possessing anterior 
non-ganglionic roots, and in not having its terminal branches 
fully developed. At stages later than n the nerve seems to arise 
from the base of the mid brain by a single large ganglionic root, 
no smaller non-ganglionic ones being visible (fig. 14, mr), 
though at stage n itself they are very prominent (fig. 13, 111). 
Besides the branches of the third nerve, mentioned above, 
there are two others in direct connection with the ganglion c.g. : 
of these the first, at stage u (fig. 11 0., NV. ¢.), is a short nerve, 
which lies along the top of the second head cavity and serves to 
' Marshall, “Morphology of Vertebrate Olfactory Organ,” * Quart. 
Journ. Micr. Sci.,’? July, 1879, pp. 300 seg. 
2 Marshall, loc. cit., pp. 78 seq. 
