THE HISTOGENESIS OF THE NERVOUS TISSUES 



315 



roots and, together with them, constitute the trunks of the spinal nerves (Fig. 



3°7)- 



The Differentiation of the Unipolar Ganglion Cells.— At first bipolar, the 

 majority of the ganglion cells become unipolar either by the unilateral growth 

 of the cell body or by the bifurcation of a single primary process. In the first case, 

 if the cytoplasm and nucleus take up an eccentric position, the two processes 

 unite in a single slender connection with the cell body (Fig. 303). The ganglion 

 cell, having one process, is now unipolar and its process is T-shaped. Many of the 

 bipolar ganglion cells persist in the adult, and others develop several secondary 

 processes and thus become multipolar 

 in form. In addition to forming the 

 spinal ganglion cells, neuroblasts of 

 the ganglion crest are believed to 

 migrate ventrally and form the sym- 

 pathetic ganglia (Fig. 307). 



The Neurone Theory. — The above ac- 

 count 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 

 Waller that if nerves are severed, the fibers 

 distal to the point of section, and thus iso- 

 lated 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 cells. 

 More recently Harrison (Amer. Jour. Anat., 



vol. 5, 1906) experimenting on amphibian larvae has shown (1) that no peripheral nerves develop 

 if the neural tube and crest are removed; (2) that isolated ganglion cells growing in clotted 

 lymph will give rise to long axis cylinder processes in the course of four or five hours. 



A second theory, supported by Schwann, Balfour, Dohrn and Bethe, assumes that the 

 nerve fibers are in part differentiated from a chain of cells, so that the neurone would represent 

 a multicellular, not a unicellular structure. Apathy and 0. Schulze modified this cell-chain 

 theory by assuming that the nerve fibers differentiate in a syncytium which intervenes between 

 the neural tube and the peripheral end organs. Held further modified this theory by assuming 

 that the proximal portions of the nerve fibers are derived from the neuroblasts and ganglion 

 cells and that these grow into a syncytium which by differentiation gives rise to the peripheral 

 portion of the fiber. This theory accords with the experiments of Bethe who found that in the 

 peripheral portions of severed nerves, functional nerve fibers were regenerated in young 

 animals. 



Fig. 303. — A portion of a spinal ganglion from a 

 human embryo of 44 mm. Golgi method (Cajal). 



