. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE OLFACTORY LOBES 407. begin to enlarge—that it to say, at about the fourth week—a slight furrow, which appears on the ventral surface of each anteriorly, marks off an area which, continuing to enlarge with the hemispheres, gradually becomes constricted off from them to form a distinct lobe- like structure, the olfactory lobe (Fig. 233, VI 3). In most of the lower mammalia these lobes reach a very considerable size, and consequently have been regarded as constituting an additional division
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE OLFACTORY LOBES 407. begin to enlarge—that it to say, at about the fourth week—a slight furrow, which appears on the ventral surface of each anteriorly, marks off an area which, continuing to enlarge with the hemispheres, gradually becomes constricted off from them to form a distinct lobe- like structure, the olfactory lobe (Fig. 233, VI 3). In most of the lower mammalia these lobes reach a very considerable size, and consequently have been regarded as constituting an additional division of the brain, known as the rhinen- cephalon, but in man they remain smaller, and although they are at first hollow, con- taining prolongations from the lateral ventricles, the cavities later on disappear and the lobes become solid. Each lobe becomes differentiated into two portions, its terminal portion becoming converted into the club-shaped struc- ture, the olfactory bulb and stalk, while its proximal portion gives rise to the olfactory tracts, the trigone, and the anterior perforated substance. Histogenesis of the Cerebral Cortex.—A satisfactory study of the histogenesis of the cortex has not yet been made. In embryos of three months a marginal velum is present and probably gives rise to the stratum zonale of the adult brain; beneath this is a cellular layer, perhaps representing the mantle layer; beneath this, again, a layer of nerve-fibers is beginning to appear, representing the white substance of the pallium; and, finally, lining the ventricle is an ependymal layer. In embryos of the fifth month, toward the in- nermost part of the second layer, cells are beginning to differentiate into the large pyramidal cells, but almost nothing is known as to the Fig. 245.—Median Longitudinal Section of the Brain oe an Embryo of the Fifth Month. ac, Anterior commissure; cc, corpus callo- sum; dg, dentate gyrus;/, fornix; i, infundib- ulum; mc, intermediate mass; si, septum pel
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