THE NERVOUS SYSTEM 



is directed centrally, the other peripherally. The centrally directed branch 

 grows into the neural tube as a sensory root fiber (Fig. 20, A, d) ; the other grows 

 peripherally as an afferent fiber of a cerebrospinal nerve. This general state- 

 ment requires some qualification. It may be that some bipolar neuroblasts 

 become unipolar by the absorption of one of the primary processes, while the 

 remaining one divides dichotomously into central and peripheral branches 

 (Streeter, 1912). It should also be noted that the cells of the sensory ganglia 



of the acoustic nerve remain bipolar 

 throughout life. 



Development of the Spinal Nerves. 

 We have traced the development 

 of the chief elements entering into 

 the formation of the cerebrospinal 

 nerves, and will now see how these 

 are combined in a typical spinal nerve. 

 The spinal ganglion, derived from 

 the neural crest, contains bipolar 

 neuroblasts, which are transformed 

 into unipolar neurons. The axon of 

 such a nerve-cell divides into a cen- 

 tral branch, running through the 

 dorsal root into the spinal cord, and 

 a peripheral branch, running distally 

 through the nerve to reach the skin 

 or other sensitive portion of the body. 

 Mingled with these afferent fibers in 

 the spinal nerves are efferent axons which have grown out from neuroblasts in 

 the basal plate of the spinal cord, througiiJJie ventral root, and are distributed 

 by way of the spinal nerve to muscles. 



So far we have dealt only with the origin of the axis-cylinders of the nerve- 

 fibers. But these soon become surrounded by protective sheaths which are also 

 ectodermal in origin. In the path of the outgrowing axons there are seen nu- 

 merous spindle-shaped ectodermal cells, which have migrated from the anlage 

 of the spinal ganglia (Harrison, 1906), and perhaps also from the neural tube 

 along the ventral roots (Held, 1909). These cells form such a prominent feat- 

 ure in a developing nerve that some workers have thought the axons differen- 

 tiate in situ from them. This theory, which has been known as the cell-chain 



Fig. 21. A section of a spinal ganglion from 

 a 44 mm. fetus, showing stages in the trans- 

 formation of bipolar neurons, A, into unipolar 

 neurons, B. Golgi method. (Cajal.) 



