936 THE AMERICAN NATURALIST. [Vor. XXXIV. 
The nerves are shown as follows : The optic nerve (JV.op.) 
still has its central cavity, which, nearer the median plane, 
opens into the third ventricle of the brain, and in the section 
resembles in shape an inverted U. On the side of the nerve 
towards the mouth there is a deep notch, — the section of the 
choroid fissure. The trigeminal ganglion (G.tvz.) is very 
large, and its trilobate form is clearly indicated by the figure. 
The lobe to which the reference line (G.77z.) runs gives off 
the ramus ophthalmicus ; the lobe nearest the jugular gives 
off the ramus maxillaris inferior, while the middle lobe 
gives off the ramus maxillaris superior. From the ganglion 
the fibres and nerve cells extend upwards to form the root 
(V.V.), which joins the hind-brain at a characteristic point, — 
namely, at the summit of the Varolian bend and where the 
hind-brain is widest (compare Figs. 2 and 3). By its great 
size and by its topographical association with the lateral apex 
of the recessus lateralis of the fourth ventricle, the trigeminal 
ganglion may always be readily identified in sections of em- 
bryos. The acustico-facial ganglia (/4c.7.) may also be readily 
determined by their typical position immediately in front of the 
otocyst (O7). But it is quite difficult to identify the four com- 
ponents of this complex structure, namely : 1?, the motor root of 
the facial nerve; 2?, the facial or geniculate ganglion ; 3°, the 
vestibular ganglion ; 4°, the cochlear ganglion. In Fig. 8 three 
divisions are shown. The large, darkly stained division, to 
which the reference line (Ac.F.) runs, and which lies nearest to 
the otocyst, is the vestibular portion of the acoustic ganglion ; 
the small, light area occupying a middle position in the inferior 
part of the complex is the motor division of the seventh nerve, 
or lateral root of the facial; it can be followed to the brain, 
which it enters as four bundles of fibres; its path of entrance 
is shown better in frontal sections (Fig. 10). Just in front of 
the facial motor root lies a second smaller dark mass, the genicu- 
late ganglion of the facial, with an upward prolongation, the 
sensory root. The ninth or glossopharyngeal nerve is repre- 
sented by the ganglion petrosum (G.petr.) and its ascending 
sensory root. This nerve may be quickly identified because it 
is the first behind the otocyst. The upper ganglion of this 
