DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 519 
A long row of closely and irregularly scattered sense-organs 
mark the path of the median trunk line—one, two, or three 
organs falling apparently as chance determines to each seg- 
ment. 
In fig. 21 the facial and vagus nerves are coloured red, the 
trigeminal and glosso-pharyngeal black. The eye muscle 
nerves are not represented. I have been unable to find the 
trochlearis in these early stages. The abducens may be easily 
followed in a slightly older embryo, and possibly already exists. 
The oculo-motorius passes from the floor of the mid-brain to 
the ramus profundus, which it meets at a point near the most 
dorsal of the branches which fig. 21 represents as leaving the 
profundus. The ganglia are represented in a flat shade as in 
the preceding reconstructions, and the position of ear, eye, 
and nose is indicated. The embryo from which the lateral 
line organs are reconstructed is the embryo drawn. The 
nerves, however, in this and in the following reconstruction 
have been traced from transverse, horizontal, and sagittal 
sections through embryos at the same stage of development as 
the embryo drawn, but killed and stained with formic acid and 
gold chloride, which I have found satisfactory and reliable re- 
agents for the topography of the nervous system, Lee (25 
p. 143) to the contrary notwithstanding. 
From the Gasserian ganglion three branches arise; the 
ramus ophthalmicus profundus, the ramus mandibularis, and a 
branch which almost immediately unites with the ramus 
buccalis facialis. The anterior part of the ganglion forms the 
posterior part of the ramus profundus, and lies close to the 
median wall of the optic vesicle. Several small branches, of 
which the two larger have been drawn, pass upward between 
the eye and the brain, connecting the profundus with the 
sensory ectoderm of the supra-orbital ridge, which, it will 
be remembered, originally contributed to the origin of the 
nerve. Between the eye and nose the profundus sends a 
branch directly outwards, and then divides into its two chief 
branches, which closely embrace the thick-walled olfactory 
vesicle. The anterior of these branches anastomoses with the 
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