. The development of the chick; an introduction to embryology . him). Nhl. 4, Neurobhist of tiie ventral horn (motor). THE NERVOUS SYSTEM 237 the dorsal root and of the sympathetic system. The first kindonly is considered here, and they are usually called the gan-glionic neuroblasts , ]:)ecause they alone remain in the spinalganglia. Like the medullary neuroblasts these neuroblasts formoutgrowths that become axis cylinder processes; but they differfrom the latter in that each ganglionic neuroblast forms twoaxones, one from each end of the spindle-shaped cells, which arearranged with their
. The development of the chick; an introduction to embryology . him). Nhl. 4, Neurobhist of tiie ventral horn (motor). THE NERVOUS SYSTEM 237 the dorsal root and of the sympathetic system. The first kindonly is considered here, and they are usually called the gan-glionic neuroblasts , ]:)ecause they alone remain in the spinalganglia. Like the medullary neuroblasts these neuroblasts formoutgrowths that become axis cylinder processes; but they differfrom the latter in that each ganglionic neuroblast forms twoaxones, one from each end of the spindle-shaped cells, which arearranged with their long axis parallel to the long axis of theganglion (Fig. 139). Thus we may distinguish a central processand a peripheral process from each neuroblast, the former grow-ing towards and the latter away from the neural tube (Fig. 139).In other words each ganglionic neuroblast is bipolar, as contrastedwith the unipolar medullary neuroblasts. The central axoneenters the dorsal zone of the neural tube, and the peripheral onegrows out into the surrounding Fig. 141. —Transverse section through the spinalcord of a 9-day chick, prepared by the methodof Golgi. (After Ramon y Cajal.)Col., Collaterals, d. R., Dorsal root. G., Gray matter. Gn., Ganglion. Nhl. 4, Neuroblast of the ventral horn (motor), v. R., Ventral root. W., White matter. In the course of the later development the cell-bodv movesto one side so that the central and peripheral branches ap]:)earnearly continuous (Fig. 141). Farther shifting of the cell-bodyproduces the characteristic form of the ganglionic nerve-cell withrounded body provided with stem fi-om whicli the central andperipheral branches pass off in opposite directions. Tiie centralprocess enters the marginal velum near its dorsal boundary and 238 THE DEVELOPMENT OF THE CHICK there bifurcates, producing two branches, one of which growstowards the head and the other towards the tail in the dorsal
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