gland_.

FIG. 22.--Dorsal and Lateral Views of the Brain of a Ratel (_Mellivora
indica_).]

_Embryology._

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 i