development is somewhat doubtful; they are probably
modifications of the lamina terminalis, but they may be secondary
adhesions between the adjacent surfaces of the cerebral hemispheres
where the cortical grey matter has not covered the white. They begin
at their antero-ventral part near the genu of the corpus callosum and
the anterior pillars of the fornix, and these are the parts which
first appear in the lower mammals. The original anterior vesicle from
which the hemispheres evaginate is composed, as already shown, of an
anterior part or telencephalon and a posterior or thalamencephalon;
the whole forming the third ventricle in the adult. Here the alar and
basal laminae are both found, but the former is the more important;
from it the optic thalami are derived, and more posteriorly the
geniculate bodies. The anterior wall, of course, is the lamina
terminalis, and from it are formed the _lamina cinerea_, the _corpus
callosum_, _fornix_ and _septum lucidum_. The roof largely remains
epithelial and is invaginated into the ventricle by the mesoderm to
form the _choroid plexuses_ of the third ventricle, but at the
posterior part it develops the _ganglia habenulae_ and the pineal
body, from a structure just in front of which both a lens and retinal
elements are derived in the lower forms. This is one great difference
between the development of this organ and that of the true eyes;
indeed it has been suggested that the pineal is an organ of thermal
sense and not the remains of a median eye at all. The floor of the
third ventricle is developed from the basal laminae, which here are
not very important and from which the _tuber cinereum_ and, until the
fourth month, single _corpus mammillare_ are developed. The
_infundibulum_ or stalk of the posterior part of the pituitary body at
first grows down in front of the _tuber cinereum_ and, according to
Gaskel's theory, represents an ancestral mouth to which the ventricles
of the brain and the central canal of the cord acted as the stomach
and intestine (_Quart. Journ. of Mic. Sci._ 31, p. 379; and _Journ. of
Phys._ v. 10, p. 153). The reason why the basal lamina is here small
is because it contains the nuclei of no cranial nerves. The anterior
and posterior commissures appear before the middle and the middle
before the _corpus callosum_, as they do in phylogeny. In connexion
with the thalamencephalon, though not re