146 



HISTOLOGY 



controlling center for the entire fiber; and any part of the fiber which is cut off from 

 the cell body undergoes degeneration. Stohr considers that Schwann (1839) had 

 the correct conception when he regarded the nerve fiber as " a secondary cell, devel- 

 oped by the coalescence of primary cells." 



Opposed to the syncytial interpretation of a peripheral fiber are the experiments 

 of Harrison, some of which have already been cited. He has shown that in the 

 tadpole the sheath cells, or neurolemma cells, which are believed by some to produce 

 the segments of the fiber which they surround, all migrate from the brain along the 

 dorsal root. If the dorsal part of the cord is removed from tadpoles, the ventral 

 roots are deprived of their sheath cells, but the fibers of the ventral roots grow out to 

 their terminations nevertheless. If the ventral part of the cord is cut from beneath 

 the dorsal part, the dorsal roots develop and have with them the sheath cells which 



Neuraxpn with 

 branching col- 

 laterals. 



FIG. 132. Two NERVE CELLS FROM THE CENTRAL NERVOUS SYSTEM. GOLGI PREPARATIONS. X 200- 



A, Cell of Deiter's type, having a neuraxon ending at a considerable distance from the cell body; B, cell of 



Golgi's type having a neuraxon with many branches ending near the cell body. 



normally would enclose the fibers of the ventral root. These sheath cells do not 

 produce nerve fibers. Therefore Harrison concludes that the peripheral fibers are 

 not syncytial. 



Recently W. H. and M. R. Lewis have caused sympathetic fibers to grow from 

 pieces of the intestine of chick embryos placed in various saline solutions. These 

 fibers show amoeboid endings. They branch freely and anastomose, but like 

 the nerve fibers from the central nervous system "they are outgrowths from nerve 

 cells and are not formed from pre-existing protoplasmic networks" (Anat. Rec., 1912, 

 vol. 6, pp. 7-31). 



Another form of syncytium would result if neurofibrils passed across the places 

 of contact between the neurones. According to Apathy, who has studied the neuro- 

 fibrils of invertebrates with special methods and faultless technique, the neurofibrils 

 pass freely from cell to cell (Mitth. Zool. Station, Naples, 1897, vol. 12, pp. 495-748). 

 It is possible that this takes place in the vertebrate nervous system also. Anastomoses 



