As already mentioned, the brain is formed from the primitive neural canal, the anterior part of which dilates into three little swellings called the anterior, middle and posterior cerebral vesicles. From the anterior, or first cerebral vesicle, at an early period spring two processes, which become the optic vesicles. These ultimately develop into the retina, and other nervous parts of the eye, with the history of which the changes occurring in them will be described.

The optic vesicles are pushed downward by two large processes growing forward from the anterior vesicle (the primitive cerebral hemispheres). The anterior part of the brain then appears to be composed of two divisions, the anterior of which is subsequently developed into the cerebral hemispheres, corpora striata, and the olfactory lobes, as a whole called prosencephalon, while the hinder part, representing the anterior vesicle, receives the name of thalamencephalon.

"he cavity of the thalamencephalon opens behind into the cavity of the middle cerebral vesicle, and in front communicates with the hollow rudiments of the cerebral hemispheres, and eventually it becomes the cavity of the third ventricle. The floor of the thalamencephalon is ultimately developed into the optic chiasma, part of the optic nerves, and the infundibulum. The latter comes in contact with a process from the mouth, uniting with which it ultimately forms the pituitary body. From the posterior part of the roof of the thalamencephalon is developed the pineal gland - a peculiar outgrowth, of unknown function, more elaborately developed in some of the lower vertebrates. The anterior part of the roof of the thalamencephalon becomes very thin, and its place is finally occupied by a thin membrane containing a vascular plexus, which persists in the roof of the third ventricle {choroidplexus). From the sides of the thalamencephalon, which become extremely thickened, are developed the optic thalami.

The primitive cerebral hemispheres first appear as two lobes growing from the anterior part of the first cerebral vesicle. The floor of these lobes thickens, and forms the corpora striata, while the roof develops into the hemispheres proper. The cavities of these lobes become the lateral ventricles, and are connected by the foramen of Monro, which at the earlier periods is very wide, but subsequently narrows to a mere slit. The cerebral hemispheres are separated at an early stage by a fold of connective tissue, which ultimately forms into the falx cerebri. The hemispheres are greatly enlarged in the backward direction, so that they quite overlap the thala-mencephalon and the parts developed from the middle cerebral vesicle. The corpus cal-losum is subsequently formed by the fusion of the juxtaposed parts of the hemispheres.

Diagram of the cerebral vesicles of the brain of a chick at the second day.

Fig. 286. Diagram of the cerebral vesicles of the brain of a chick at the second day. (Cad/at.) 1, 2, 3. Cerebral vesicles, o. Optic vesicles.

Diagram of a vertical longitudinal section of the developing brain of a vertebrate animal

Fig. 287. Diagram of a vertical longitudinal section of the developing brain of a vertebrate animal, showing the relation of the three cerebral vesicles to the different parts of the adult brain, {Huxley).

Olf. Olfactory lobes. Fm. Foramen of Monro. Cs. Corpus striatum. Th. Optic thalamus. Pn. Pineal gland. M.b. Mid brain. Cb. Cerebellum. Mo. Medulla oblongata. Hmp. Cerebral hemispheres Th.E. Thalamencephalon. Py. Pituitary body. CQ. Corpora Quadrigemina C.C. Crura cerebri. PV. Pons Varolii. 1. - XII. Regions from which spring the cranial nerves. 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth ventricle.

Diagram of a horizontal section of a vertebrate brain.

Fig. 288. Diagram of a horizontal section of a vertebrate brain. (Huxley).

Olf. Olfactory lobes. Lt. Lamina terminalis. Cs. Corpus striatum. Th. Optic thalamus. Pn. Pineal gland. Mb. Mid brain. Cb. Cerebellum. Mo. Medulla oblongata. 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth ventricle. + Iter a tertio ad quartum ventriculum. FM. Foramen of Monro. II. Optic nerves.

From the anterior part of the cerebral hemispheres arise two prolongations, which develop into the olfactory bulbs; these grow forward, and soon lose their cavities, which at first communicated with those of the ventricles.