. The development of the chick; an introduction to embryology . 44. — Transverse section through the cervical swellingof the spinal cord of a chick, middle of the fifth day. (AfterV. Kupffer.)bl. Blood vessel. C. a., .terior commissure. C, Cen-tral canal, d., Group of axones at the level of the , Ependyma. Nbl., Neuroblasts. V. Ventral colunm ofwhite matter. 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
. The development of the chick; an introduction to embryology . 44. — Transverse section through the cervical swellingof the spinal cord of a chick, middle of the fifth day. (AfterV. Kupffer.)bl. Blood vessel. C. a., .terior commissure. C, Cen-tral canal, d., Group of axones at the level of the , Ependyma. Nbl., Neuroblasts. V. Ventral colunm ofwhite matter. 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 cohimnsof the two sides are still sejiarated in the median line by a broadseptum of ependymal cells. Later (Fig. 146, twelve dax^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 them. ■^fi- GrM. y^:?^ Centr 7n N Fig. 1 1.). rr;ins\(Tsc section tlii-diiuli tlic spinal cord, and the eighteenthspinal ganglion of an eight-day , Centrum of v(^rte})ra. d. R., Dorsal root. Ep., Ependyma. Gn.,Hpinal Ganglion. Gn. symj)., Synij)athetic ganglion. Gr. M., Gray matter,m. N., Motor nucleus. \\. com., Hanuis connuunicans. R. d., Ramus dor-salis. R. 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 liccomiiig the prac-tically circular central canal of the fully formed cord. Up tothe sixth day it is elongated dorso-ventrally, usually narrowestin the middle with both doi-sal and \(MitfaI enlargements. Aliout THE NERVOUS SYSTEM 243 the seventh day the dorsal ]wrtion begins to be obliterated byfusion of the ependymal cells, and is thiis reduced to an epen-dymal septum.
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