DEVELOPMENT OF NERVE-ROOTS 



99 







' 



** 



conl of ectoderm- cells. Differentiation next occurs in the ganglion exactly as it 

 does in the neural tube, and bundles of fibres appear among the cells. These 

 extend through the cellular strand (fig. 136) and enter the wall of the neural tube, 

 where they form a special bundle in the reticular zone (see p. 101). From the 

 opposite end of the ganglion, bundles of fibres extend peripherally and join the 

 fibres of the ventral root 

 to form the mixed nerve- 

 trunk. The picture pre- 

 sented by a preparation 

 of a developing spinal 

 ganglion stained by the 

 Golgi method shows that 

 the bundles of fibres are 

 proximal and distal pro- 

 cesses of bipolar neuro- 

 blasts, or, as Held be- 

 lieves, bundles of neuro- 

 fibrillee extending proxi- 

 mally and distally from the 

 ganglion- cells through a 

 syncytium into which the 

 primitive ganglion has 

 been resolved. The mixed 

 nerve- trunks are at first 

 composed of rather loosely 

 arranged fibrils, but soon 

 they appear as tracts of 

 compactly arranged bun- 

 dles of fibres which, traced 

 outwards, divide up, and 

 finally have the appearance 

 of ending freely as single 

 fibres, on which a terminal 

 enlargement is shown by 

 the Golgi method of stain- 

 ing. This terminal enlarge- 

 ment is highly character- * : % \n* 

 istic of developing nerve- if 

 fibres. 



The developing nerve- 

 paths are studded with 

 nuclei. It is generally 



agreed that these represent the nuclei of the future sheath of Schwann, but 

 opinion is divided as to their origin. 



It is not possible here to give more than the briefest notice of the main hypotheses which 

 have been advanced regarding the mode of formation and growth of nerve-fibres. They may 

 be represented in tabular form thus : 



I. Outgrowth theory (Bidder, Kupffer, His, Cajal, Kolliker, Lenhossek). Each nerve-fibre 

 is the process of a nerve-cell (neuroblast) which by free terminal growth seeks out its proper 

 end-organ and comes secondarily into relation with it. In the case of the bipolar neuroblast? 

 there is growth in two directions. 



II. Cell-chain theory (Balfour, Marshall, Dohrn, van Wijhe). Each nerve is the product of 

 a chain of medullary ectoderm-cells, which extends by proliferation and establishes a secondary 

 nexus between central and end organ. 



H 2 



* & a* 



- 





I 





,>;::* 



* $\> % 



V\ 



.;>,x.. 



V 



1 



* 



FIG. 136. SPINAL GANGLION AND ANTERIOR NERVE-ROOT IN A 



RABBIT- EMBRYO OP THE TWELFTH DAY. (T. H. Bryce.) 



