slight enlargement of the anterior end
of the cord. The single cavity seen in Amphioxus is here subdivided
into three: an anterior or prosencephalon, a middle or mesencephalon,
and a hinder or rhombencephalon. The rhombencephalon has a very slight
transverse thickening in the fore-part of its roof, this is the
rudimentary cerebellum (_Cer._); the rest of this part of the brain is
taken up by the large medulla, the cavity of which is the _fossa
rhomboidalis_ or fourth ventricle. This fossa is roofed over by the
epithelium lining the cavity of the ventricle, by pia mater and
blood-vessels constituting a choroid plexus (fig. 16, B). The fourth
ventricle communicates with the parts in front by means of a passage
known as the aqueduct of Sylvius.

The mesencephalon or mid-brain, when looked at from the dorsal
surface, shows a pair of large hollow swellings, the optic lobes or
_corpora bigemina_. Their cavities open out from the aqueduct of
Sylvius, and from the nervous tissue in their walls the optic nerves
derive their fibres. From the front of the prosencephalon or anterior
vesicle the olfactory nerves come off, and at the base of each of
these are two hollow swellings; the larger and more anterior is the
olfactory bulb, the smaller and more posterior the cerebral
hemisphere. Both these swellings must be regarded as lateral
outgrowths from the blind front end of the original single vesicle of
the brain as seen in Amphioxus, and from the anterior subdivision or
prosencephalon in the lamprey. The anterior vesicle, however, is now
again subdivided, and that part from which the cerebral hemispheres
bud out, and the hemispheres themselves, is called the telencephalon,
while the posterior part of the original prosencephalon is known as
the thalamencephalon, or more rarely the diencephalon. On the dorsal
surface of the thalamencephalon are two nervous masses called the
ganglia habenulae; the right is much larger than the left, and from it
a stalk runs forward and upward to end in the vestigial pineal body
(or epiphysis), which contains rudiments of a pigmented retina and of
a lens, and which is usually regarded as the remains of one of a pair
of median eyes, though it has been suggested that it may be an organ
for the appreciation of temperature. From the small left ganglion
habenulae a still more rudimentary pineal stalk projects, and there
are signs of a t