THE HISTOGENESIS OF THE NERVOUS TISSUES 



3°5 



connected at first by bridges of cells which later disappear. In the hind-brain 

 region certain ganglia of the cerebral nerves develop from the crest but are not seg- 

 mentally arranged. 



The Differentiation of the Afferent Neurones.— The cells of the spinal gangha 

 differentiate into (1) ganglion cells, and (2) supporting cells, groups which are 

 comparable to the neuroblasts and spongioblasts of the neural tube. The neuro- 

 blasts of the ganglia become fusiform and develop a primary process at either pole ; 

 thus these neurones are of the bipolar type (Fig. 307 d). The centrally directed 

 processes of the ganglion cells converge and by elongation form the dorsal roots. 

 They penetrate the dorso-lateral wall 

 of the neural tube, bifurcate, and course 

 cranially and caudally in the marginal 

 layer of the spinal cord. By means of 

 branched processes they come in con- 

 tact with the neurones of the mantle 

 layer. The peripheral processes of the 

 ganghon cells, as the dorsal spinal roots, 

 join the ventral roots, and, together 

 with them, constitute the trunks of the 

 spinal nerves (Fig. 325). 



The Differentiation of the Unipolar 

 Ganglion Cells. — At first bipolar, the 

 majority of the ganglion cells become 

 unipolar either by the fusion of the two 

 primary processes or by the bifurcation 

 of a single process (Fig. 310). The pro- 

 cess of the unipolar ganglion is now 

 T-shaped. Many of the bipolar ganglion cells persist in the adult, while others 

 develop several secondary processes and thus become multipolar in form. In 

 addition to forming the spinal ganglion cells, neuroblasts of the gangKon crest 

 are beHeved to migrate ventrally and form the sympathetic ganglia (Fig. 325). 



The Neurone Theory. — The above account of the development of the nerve fibers is 

 the one generally accepted at the present time. It assumes that the axis cylinders of all 

 nerve fibers are formed as outgrowths, each from a single cell, an hypothesis first promulgated 

 by His. The embryological evidence is supported by experiment. It has long been known 

 from the work of WaUer that if nerves are severed, the fibers distal to the point of section, 

 and thus isolated from their nerve cells, will degenerate; also, that regeneration will take 

 place from the central stumps of cut nerves, the fibers of which are still connected with their 



Fig. 310. — A portion of a spinal ganglion from a 

 fetus of 44 mm. Golgi method (Cajal). 



