ally belonging to it, may be
mentioned the anterior lobes of the pituitary body; these begin as an
upward _diverticulum_ from the posterior wall of the primitive pharynx
or _stomatodaeum_ about the fourth week. This _pouch of Rathke_, as it
is called, becomes nipped off by the developing base of the skull, and
its bifid blind end meets and becomes applied to the posterior part of
the body, which comes down from the brain. In the mesencephalon the
alar laminae form the _corpora quadrigemina_; these at first are
bigeminal and hollow as they are in the lower vertebrates. The basal
laminae thicken to form the _crura cerebri_. In the rhombencephalon
the division into basal and alar laminae is better marked than in any
other part; there is a definite groove inside the fourth ventricle,
which remains in the adult as the superior and inferior _fovea_ and
which marks the separation between the two laminae. In the basal
laminae are found the deep origins of most of the motor cranial
nerves, while those of the sensory are situated in the alar laminae.
The roof of the fourth ventricle widens out very much and remains
largely epithelial as the superior and inferior medullary vela. The
cerebellum develops in the anterior part of the roof of the
rhombencephalon as two lateral rudiments which unite in the mid line
and so form a transverse bar similar to that seen in the adult
lamprey; at the end of the second month the flocculus and
paraflocculus become marked, and later on a series of transverse
fissures occur dividing the various lobes. Of the cerebellar peduncles
the inferior develops first (third month), then the middle forming the
_pons_ (fourth month), and lastly the _superior_ (fifth month) (Elliot
Smith, _Review of Neurology and Psychiatry_, October 1903; W. Kuithan,
"Die Entwicklung des Kleinhirns bei Saugetieren," _Munchener Med.
Abhandl._, 1895; B. Stroud, "Mammalian cerebellum," _Journ. of Comp.
Neurology_, 1895). Much of our knowledge of the tracts of fibres in
the brain is due to the fact that they acquire their white sheaths at
different stages of development, some long after birth.

For further details and references see Quain's _Anat._ vol. i. (1908);
Minot's _Human Embryology_ (New York); W. His, _Anat. menschlicher
Embryonen_ (Leipzig, 1881); Marshall's _Vertebrate Embryology_;
Kolliker, _Grundriss der Entwickelungsgeschichte_ (Leipzig, 1880); A.