. The development of the chick; an introduction to embryology . ^- blV d/. y Fig. 144. — Transverse section through the cervical swelHngof the spinal cord of a chick, middle of the fifth day. (AfterV. Kupffer.)bl. v., Blood vessel. C. a., Anterior commissure. C, Cen-tral canal, d., Group of axones at the levelof the dorsal root^Ep., Ependyma. Nbl., Neuroblasts,white matter. V. Ventral column of ventral roots. The dorsal column begins first as a bundle offibers at the entrance of the fibers of the dorsal root (Fig. 144).Subsequently, other fibers come in this region and graduallyextend tow
. The development of the chick; an introduction to embryology . ^- blV d/. y Fig. 144. — Transverse section through the cervical swelHngof the spinal cord of a chick, middle of the fifth day. (AfterV. Kupffer.)bl. v., Blood vessel. C. a., Anterior commissure. C, Cen-tral canal, d., Group of axones at the levelof the dorsal root^Ep., Ependyma. Nbl., Neuroblasts,white matter. V. Ventral column of ventral roots. The dorsal column begins first as a bundle offibers at the entrance of the fibers of the dorsal root (Fig. 144).Subsequently, other fibers come in this region and graduallyextend towards the dorsal middle line, displacing the ependyma 242 THE DEVELOPMENT OF THE CHICK and gray matter (Fig. 145, eight days), but the dorsal columnsof the two sides are still separated in the median line by a broadseptum of ependymal cells. Later (Fig. 146, twelve da\^s) thisseptum becomes very narrow, and the accumulation of fibers inthe dorsal columns causes the latter to project on each side ofthe middle line, thus forming an actual fissure between Fig. 145. — Transverse section through the spinal cord, and the eighteenthspinal ganglion of an eight-day , Centrum of vertebra, d. R., Dorsal root. Ep., Ependyma. Gn.,Spinal Ganglion. Gn. symp., Sympathetic ganglion. Gr. M., Gray matter,m. N., Motor nucleus. R. com., Ramus communicans. R. d., Ramus dor-salis. R. v., Ramus ventralis. Sp., Spinous process of vertebra, v. R.,Ventral root. Wh. M., White matter. Central Canal and Fissures of the Cord. The central canalpasses through a series of changes of form in becoming the prac-tically circular central canal of the fully formed cord. Lp tothe sixth day it is elongated dorso-ventrally, usually narrowestin the middle with both dorsal and ventral enlargements. About THE NERVOUS SYSTEM 243 the seventh day the dorsal portion begins to be ol^hterated byfusion of the ependymal cells, and is thus reduced to an epen-dymal septum. On the eighth day this process has inv
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