[Illustration: From _Cat. R.C.S. England_.
FIG. 23.--Lateral view of cerebral hemisphere of Gorilla (_Anthropopithecus gorilla_).]
It will now be seen that the original neural canal, which is lined by ciliated epithelium, forms the ventricles of the brain, while superficial to this epithelium (_ependyma_) the grey and white matter is subsequently formed. It has been shown by His that the whole neural tube may be divided into _dorsal_ or _alar_, and _ventral_ or _basal_ laminae, and, as the cerebral hemispheres bud out from the dorsal part of the anterior primary vesicle, they consist entirely of alar laminae. The most characteristic feature of the human and anthropoid brain is the rapid and great expansion of these hemispheres, especially in a backward direction, so that the mesencephalon and metencephalon are hidden by them from above at the seventh month of intra-uterine life. At first the foramina of Munro form a communication not only between the third and lateral ventricles, but between the two lateral ventricles, so that the cavity of each hemisphere is continuous with that of the other; soon, however, a median longitudinal fissure forms, into which the mesoderm grows to form the falx, and so the foramina of Munro are constricted into a V-shaped canal. In the floor of the hemispheres the corpora striata are developed at an early date by a multiplication of nerve cells, and on the external surface a depression, called the _Sylvian fossa_, marks the position of the future central lobe, which is afterwards hidden as the lips of the fossa (_opercula_) gradually close in on it to form the Sylvian fissure. The real fissures are complete infoldings of the whole thickness of the vesicular wall and produce swellings in the cavity. Some of them, like the choroidal on the mesial surface, are developed very early, while the vesicle is little more than epithelial, and contain between their walls an inpushing of mesoderm to form the choroid plexus. Others, like the hippocampal and calcarine, appear in the second and third months and correspond to invaginations of the nervous tissue, the hippocampus major and minor. The sulci appear later than the fissures and do not affect the internal cavity; they are due to the rapid growth of the cortex in certain areas. The corpus callosum and fornix appear about the third month and their 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.