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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
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