DEVELOPMENT OF THE NERVOUS TISSUES. 



lOII 



gives rise to the definite supporting tissue, the neurogha. According to Hardesty, the glia-fibres 

 arise within the syncytial tissue independently of the neuroglia cells, a view in direct opposition 

 to the observations of Rubaschkin, who attributes to the descendants of the spongioblasts, the 

 gliagenetic cells, a positive role in the production of the fibres. Accepting the conclusions of 

 the last-named investigator, the successive stages of the cells concerned in the productionof 

 the general neurogliar tissue are represented by the spongioblasts, the gliogenetic cells, the 

 astrocytes, and, finally, the glia cells. The primary ependymal clemetits are succeeded by the 

 epithelium which lines the ventricles and the central canal of the spinal cord. Their periph- 

 erally directed processes are in large part transformed into glia-fibres and thus, along with the 

 processes of the spider cells, contribute to the formation of the neurogliar felt-work. The 

 accompanying illustration (Fig. 857), taken from Hardesty's paper, affords an instructive 

 comparison of the appearance of the young supporting tissue after true staining with approved 

 reagents (Benda) and after silver precipitation methods (Goigi) upon which so much reliance 

 has been placed. The silver picture shows the classic long neurogliar fibres extending the 

 entire thickness, but fails to reveal the wealth of-supportmg tissue and nuclei. To what 

 extent the mesoblastic ingrowths that follow the penetrating young blood-vessels into the neural 

 wall take part in the production of the distinctive neurogliar framework is admittedly difficult to 

 determine (Hardesty) ; that such tissue, however, contributes to the support of the nervous 

 elements is certain. 



Histogenesis of the Neurones. — The neuroblasts are distinguishable with certainty from the 

 spongioblasts as soon as they are provided with nerve-processes. The latter appear as out- 

 growths from the pointed and 

 peripherally directed ends of the 

 developing nerve-cells, invade the 



marginal zone, and later emerge 

 from the wall of the immature 

 cord as the ventral or anterior 

 root-fibres of the spinal nerves 

 (Fig. 858). The deeper tint of 

 their distal ends after staining, 

 their tendency to collect in con- 

 verging groups, and the uniform 

 width of the outgrowing nerve- 

 processes are distinctive charac- 

 teristics of the neuroblasts ( His ^ ). 

 The first, and for a considerable 

 time the only processes with which 

 the neurones are provided cor- 

 respond to the axones that be- 

 come the axis-cylinders of the 

 efferent (motor) nerves. Subse- 

 quently other processes, the den- 

 drites, grow out in various direc- 

 tions from the cell-bodies of the 

 young neurones. 



Development of the Peripheral Nerves 





Neuroblasts 



Efferent axones 







Portion of spinal cord of human embryo, showing development of 

 ventral root-axones as outgrowths from ventral neuroblasts. X 2P^- 

 (After His.) 



According to the teaching of His, accepted by 

 most anatomists, the axis-cylinder of the entire future nerve-fibre is formed by the peripheral 

 growth of the original nerve-process of the neuroblast. The assumed development of the nerve- 

 fibre by the union of» a number of segments ( Balfour, Dohrn, and others, and, more recently, 

 Bethe and O. Schultze) is not in accord with renewed investigations, and the findings upon 

 which the composite theory of the fibre is based arc; open to different interpretation (Kolliker, 

 Retzius). 



According to Bardeen,^ the development of the peripheral spinal nerves is briefly as follows: 

 The motor neuroblasts and the sensory spinal ganglion-cells send out processes of considerable 

 thickness, all of which soon begin to give rise at their extremities to groups of fibrillin, which 

 increase in thickness and length and, in turn, at their extremities give rise to new groups of 

 fibrils. At first these proceed as naked bundles, but soon become surrounded with nucleated 

 fusiform sheath-cells which thus enclose the early embryonic nerve, and may contain hundreds 

 of fibrillae. After a nerve has become distended by ingrowth of new fibrils from behind, the 

 proliferating sheath cells begin to wander from the periphery in among the fibrillae and give rise 

 by anastomosis of their processes to a net-work that divides the original fasciculus into a number 

 of secondary bundles. The intrafascicular cells increase rapidly, the process of subdivision 



' Die Entwickelung des menschlichen Gehirns, 1904. 

 'Amer. Journal of Anatomy, vol. ii., 1903. 



