Whether the cortical cells are developed in situ or migrate from the matrix near the ventricle to their adult position, is not yet determined. E. Lindon Mellus presents evidence in support of the migration theory. This is also supported by the analogous development in the spinal cord. It has been claimed by Streeter and others that there is no migration after four and a half months; but the findings of Mellus show the corona radiata filled with neuroblasts in the last two months of pregnancy and in the early extra-uterine life. He concludes that the cortical neurones do not arise in situ. They originate in the matrix and continue to form there and migrate to their positions in the cortex until a short time after birth, when the matrix is exhausted. Not all cortical cells are in position at birth. Mellus estimates the number of neurones ultimately located in the cortex at roo,000 per cu. mm. (Am. Jour. Anat., Vol. 14).
Visceral, Somatic and Association Areas.—In the lower fishes (ganoids and teleosts) the telencephalon is little more than olfactory bulb and nucleus of the terminal nerve; the division into hemispheres is merely indicated The lateral evagination, which forms the hemisphere, becomes progress ively more complete in amphibians, in reptiles and in mammals. In man the telencephalon medium is very small, bounding the aula of the third ventricle, while the hemisphere is ponderous in comparison. The fore brain, from the lowest forms to the highest, is differentiated into visceral and somatic areas (J. B. Johnston).
The visceral area is made up of the olfactory, gustatory and general visceral centers. It receives the second and third order neurones from the olfactory bulb and the tractus pallii from the hypothalamus; and it possesses certain commissures, the fornix and a characteristic structure.
Its parts develop rich associations with one another and then are cor related with the visual center and the motor mechanism employed to obtain food. The visceral cortex is in this manner elaborated into a
large annular gyrus, the hippocampal formation (formerly called the arehi pallium). It is large in reptiles, forming, with the medial and lateral olfactory gyri, a complete ring; but in mammals the development of the corpus callosum destroys the superior part, leaving only the rem nants—subcallosal and supracallosal gyri—in that region. According to J. B. Johnston the hippocampal formation is of the same age as the somatic area; hence, it, may be called rhinopallium but not archipallium.
The somatic area of the fore-brain is made up of the centers of cutaneous and muscular sensibility, and the visual, static and acustic centers. It receives the nervus terminalis and the thalamo-cortical fibers, bearing common sensory impulses; also, the fibers of the visual, vestibular and cochlear paths. Many automatic centers located primarily in the thala mus and striatum are shifted during development to the cortex, the func tions of the lower centers therely being reduced. The various cortical centers enlarge by growth and internal association. They become cor related one with another, and the somatic with the visceral centers. As a result, certain correlation centers are formed between the somatic and visceral centers which correspond to the association areas of Flechsig. These correlation centers form common clearing houses where afferent impulses are received upon an equality and interact upon one another, modifying, annulling and combining them into new forms. Hence, the response to stimuli in vertebrates is modified and rationalized; it is not a simple reflex (J. B. Johnston, Anat. Rec., Vol. 4).
The aggregation of the centers of countless reflex mechanisms, together with their elaboration and internal association, constitutes the sensory and the motor cortex, and the correlation mechanisms make up the psychic-sensory, the psychic-motor and the higher psychic cortex.