_auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres.
_ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._
Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes.
_md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._
Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus.
_pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland.
_pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the
roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve.
_x._ Rudiment of septum which will grow backwards and divide the unpaired
cerebral rudiment into the two hemispheres.
Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium
embryo belonging to stage L. Zeiss a, ocul. 1.
1_a_ is taken slightly to one side of the middle line, and shews the
general features of the brain, and more especially the infundibulum (_in_)
and pituitary body (_pt_).
1_b_ is through the median line of the pineal gland.
1_c_ is through the median line of the base of the brain, and shews the
notochord (_ch_) and pituitary body (_pt_); the latter still communicating
with the mouth. It also shews the wide opening of the infundibulum in the
middle line into the base of the brain.
Fig. 2. Section through the unpaired cerebral rudiment during stage O, to
shew the origin of the olfactory lobe and the olfactory nerve. The latter
is seen to divide into numerous branches, one of which passes into each
Schneiderian fold. At its origin are numerous ganglion cells represented by
dots. Zeiss a, ocul. 2.
Fig. 3. Horizontal section through the three lobes of the brain during
stage O. Zeiss a, ocul. 2.
The figure shews (1) the very slight indications which have appeared by
this stage of an ingrowth to divide the cerebral rudiment into two lobes
(_x_): (2) the optic thalami united by a posterior commissure, and on one
side joining the base of the mid-brain, and behind them the pineal gland:
(3) the thin posterior wall of the cerebral rudiment with folds projecting
into the cerebral cavity.
Figs. 4_a_, 4_b_, 4_c_. Views from the side, from above, and from below, of
a brain of Scyllium canicula during stage P. In the view from the side the
eye (_op_) has not been removed.
The bilobed appearance both of the mid-brain and cerebellum should be
noticed.
Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage
P. Zeiss a, ocul. 2.
There should be noticed (1) the increase in the flexure of the brain
accompanying a rectification of the cranial axis; (2) the elongated pineal
gland, and (3) the structure of the optic thalamus.
Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of
a brain of Scyllium stellare during a slightly later stage than Q.
Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a
Scyllium embryo during stage Q. Zeiss a, ocul. 2.
7_a_ cuts the hind part of the brain nearly through the middle line; while
7_b_ cuts the cerebral hemispheres and pineal gland through the middle.
In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform
tracts (_rt_) into the cerebellum (3), and the rudiments of the tela
vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the
cerebral hemispheres (1), the pineal gland (2), and the relations of the
optic thalami (3) are shewn.
Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an
embryo slightly older than Q. Zeiss a, ocul. 1.
8_a_ passes through the cerebral hemispheres at their junction with the
olfactory lobes. On the right side is seen the olfactory nerve coming off
from the olfactory lobe. At the dorsal side of the hemispheres is seen the
pineal gland (_pn_).
8_b_ passes through the mid-brain now slightly bilobed, and the opening
into the infundibulum (_in_). At the base of the section are seen the optic
nerves and their chiasma.
8_c_ passes through the opening from the ventricle of the mid-brain into
that of the cerebellum. Below the optic lobes is seen the infundibulum with
the rudiments of the sacci vasculosi.
8_d_ passes through the front end of the medulla, and shews the roots of
the seventh pair of nerves, and the overlapping of the medulla by the
cerebellum.