[Illustration: From _Cat. R.C.S. England_.
FIG. 22.--Dorsal and Lateral Views of the Brain of a Ratel (_Mellivora indica_).]
The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this _medullary groove_ unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of _prosencephalon_, _mesencephalon_ and _rhombencephalon_ are now usually given. The simple tubular brain we have seen as a permanent arrangement in Amphioxus, but the stage of the three vesicles is a transitory one, and is not found in the adult of any existing animal. From the sides of the prosencephalon, the optic vesicles grow out before the neural tube is completely closed, and eventually form the optic nerves and retinae, while, soon after this, the cerebral hemispheres bulge from the antero-dorsal part of the first primary vesicle, their points of evagination being the _foramina of Munro_. From the ventral parts of these cerebral hemispheres the olfactory lobes are constricted off, while just behind the openings of the foramina of Munro a constriction occurs which divides the prosencephalon into two secondary vesicles, the anterior of which, containing the foramina of Munro, is called the _telencephalon_, while the posterior is the _thalamencephalon_ or _diencephalon_. A constriction also occurs in the hind vesicle or _rhombencephalon_, dividing it into an anterior part, the _metencephalon_, from which the cerebellum is developed, and a posterior or _myelencephalon_, the primitive _medulla oblongata_. At this stage the general resemblance of the brain to that of the lamprey is striking.
Before the secondary constrictions occur three vertical flexures begin to form. The first is known as the _cephalic_, and is caused by the prosencephalon bending sharply downward, below and in front of the mesencephalon. The second is the _cervical_, and marks the place where the brain ends and the spinal cord begins; the concavity of this flexure is ventral. The third to appear has a ventral convexity and is known as the _pontine_, since it marks the site of the future _pons Varolii_; it resembles the permanent flexure in the reptilian brain.