The inner cell-layer of Bolton, which corresponds to the fusiform cells and the internal pyramids of Campbell and Cajal, is the first to differen tiate; and within it the neuroblasts first attain their full size and form. It is said to be the best developed cortical lamina in all mammals (G. A. Watson). It is thicker in the association areas than in the receptive regions. Its thickness is equal in man and the rhesus monkey; but it is thicker in the dog than in man. In the central gyri of the 18 weeks' foetus of Bolton, it is faintly indicated and the Betz-cells are the only ones out of the embryonic stage. The inner cell-lamina is the fundamental layer. By inference it presides over the organic and instinctive activities.
The inner line of Baillarger, a microscopic plexus of tangential fibers, may be descried just above the inner cell-layer at the sixth month. The line is well developed only in the association areas and does not become definite until the seventh or eighth month. It occupies the level of the internal large pyramids of Campbell. Among its fibers lie the giant pyri form pyramidal cells of Betz in the motor cortex and the great pyramids of Meynert in the visuo-sensory cortex. The Betz-cells are present as large irregular pyramids in the r8 weeks' foetus and very clearly map out the motor cortex. They measure at this time 29-36p in height and 7y in width; their nuclei are 5.5µ in diameter. The Betz-cells give rise to the pyramidal tract. They are purely motor. The inner line of Baillarger appears to form a part of the instinctive mechanism belonging chiefly to the inner cell-layer (fusiform cells and internal large pyramids).
The middle cell-layer of Bolton is the stellate layer of Campbell (layer V). It is represented only by a few scattered cells in the anterior central gyms; but in all sensory cortex, except the olfactory, it is a prominent lamina. In the visuo-sensory cortex it is exceptionally developed. In that it is divided into two laminae at the eighteenth week by the outer line of Baillarger (line of Gennari); though at that time all its cells are very embryonic and diffuse in arrangement. It is thicker in the visuo-sensory than in the visuo-psychic area and is very poorly developed in those blind from birth. In the prefrontal area it varies in thickness inversely with the degree of amentia and dementia. It is very well developed in the sensory
projection areas of all vertebrates. Constituting the receptive lamina of the cortex, its function seems to be the transformation of afferent impulses. The optic radiation enters the stellate layer in the visuo-sensory cortex and forms the greater part of the outer line of Baillarger, the line of Gennari, which is a prominent naked eye feature in that region. Outside the visual cortex there is but one layer of stellate cells and the outer line of Baillarger is microscopic and very faint, except in association regions; it lies at the level of the large external pyramids.
The outer cell-layer of Bolton embraces the small, medium and external large pyramids of Campbell. It is poorly developed in all mammals below man; it thickens and increases in differentiation with the ascent of the mammalian scale, and is the last layer evolved in man. At birth it pos sesses only half its normal thickness and its cells are largely embryonic. It is one-fourth thicker than all the underlying laminae in adult man; it just equals them in the rhesus monkey, and is but one-third as thick as the subjacent layers in the dog. Its thickness varies inversely with the degree of amentia and dementia; the subevolution and degeneration reverse the order of development: when the defect or the degeneration is only slight it affects the small pyramids; then, the medium pyramids, if more severe; and, finally, the large pyramids in the deep part of the layer. The outer cell-layer shows marked progressive improvement in the development of the individual pyramids as we pass from dog to rhesus and from rhesus to man. The deeper pyramids differentiate first. Gradually, through a period of years, the evolution approaches the surface. The small closely packed embryonic neuroblasts become arranged into irregular columns about the eighteenth week in the motor and common sensory zones, elsewhere this occurs later; the cytoplasm increases in bulk and develops its charac teristic constituents; the dendrites multiply and extend their arboriza tions; the pyramidal form is gradually assumed, and the cells recede from each other as the cortex expands and folds itself into gyri. The outer cell-layer in depth and differentiation characterizes man; it is associative and psychic in function.