. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 386 THE BRAIN A " ; t ffe V/tt 1 —H /»// \-mt certain that the lateral and anterior cerebro-spinal (pyramidal) fasciculi are composed of fibers which grow downward in the meshes of the marginal velum from neuroblasts situated in the cerebral cortex, while the cerebellospinal (direct cerebellar) fasciculi and the fibers of the ground-bundles have their origin from cells of the mantle layer of the cord. The myelination of the fibers of the spinal cord begins between the fifth and sixth mon
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 386 THE BRAIN A " ; t ffe V/tt 1 —H /»// \-mt certain that the lateral and anterior cerebro-spinal (pyramidal) fasciculi are composed of fibers which grow downward in the meshes of the marginal velum from neuroblasts situated in the cerebral cortex, while the cerebellospinal (direct cerebellar) fasciculi and the fibers of the ground-bundles have their origin from cells of the mantle layer of the cord. The myelination of the fibers of the spinal cord begins between the fifth and sixth months and appears first in the funiculi cuneati, and about a month later in the funiculi graciles. The myelination of the great motor paths, the lateral and anterior cerebro-spinal fas- ciculi, is the last to develop, appearing to- ward the end of the ninth month of fetal life. The Development of the Brain. —The enlargement of the anterior portion of the medullary canal does not take place quite uniformly, but is less along two transverse lines than else where, so that the brain region early becomes divided into three primary vesicles which undergo further differ- entiation as follows. Upon each side of the anterior vesicle an evagination appears and becomes converted into a club-shaped structure attached to the ventral portion of the vesicle by a pedicle. These evaginations (Fig. 232, op) are known as the optic evag- inations, and being concerned in the formation of the eye will be considered in the succeeding chapter. After their formation the antero-lateral portions of the vesicle become bulged out into two protuberances (h) which rapidly increase in size and give rise, eventually to the two cerebral hemispheres, which form, together with the portion of the vesicle which lies between them, what is termed the telencephalon or fore- brain, the remainder of the vesicle giving rise to what is known as. Fig. 232.—Reconstruction of the Brain of an Embryo of MM. h, Hemisp
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