VI.] THE SPINAL GANGLIA. 153 



The fibres of the nerve can easily be traced through the ganglion. In 

 section they appear to have a somewhao wavy course, and by interlacing divide 

 the ganglion into a number of elongated areas in each of which is a row of 

 nuclei. In sections of the sixth day it is not possible to trace a connection 

 between the nerve-fibres and the ceils. The nuclei are most numerous at 

 the lower ends of the ganglia. 



On the seventh day, the nuclei have become larger, and where the outline 

 of a cell can be distinctly seen it is generally somewhat angular. In sections 

 it is still on the seventh day not possible to trace any connection between the 

 cells and the nerve fibres. 



Eemak (op. cit.) speaks of the ganglia being composed of non-nucleated 

 spheres, and Lockhart Clarke {Philosophical Transactions, 1862) also describes 

 the ganglia-cells as "ceLs or nuclei" which are at first mere rounded masses 

 of protoplasm, and do not acquire a nucleus till a later period. Both of these 

 statements are according to our observations incorrect. 



In the later stages accordingto Lockhart Clarke's account (/of. cit.) the cells 

 of the ganglia send out pmces.ses wiiich anastomose together into a fine 

 network. The cells also become connected with the nerve fibres, which can 

 sometimes be seen to divide in the ganglion into a fine brush-like bundle of 

 fibrilte. At this time the cells possess a distinct nucleus and nucleolus. 

 These changes he describes as completed by the ninth day of incubation. 



His believes that the spinal nerves are derived from downward prolonga- 

 tions of the superficial epiblast descending between the protovertebris. This 

 view has not been corroborated by subsequent observers. 



12. The remaining portions of the protovertebraj form- 

 ing the primary vertebra or membranous vertebral column 

 spoken of in the last paragraph, are converted into the per- 

 manent vertebras ; but their conversion is complicated by a 

 remarkable new or secondary segmentation of the whole 

 vertebral column. 



On the fourth day, the transparent lines marking the 

 fore and aft limits of the protovertebrse are still distinctly 

 visible. On the fifth day, however, they disappear, so that 

 the whole vertebral column becomes fused into a homoge- 

 neous mass whose division into vertebrae is only indicated by 

 the series of ganglia. This fusion, which does not extend to 

 the muscle-plates in which the primary lines of division still 

 remain visible, is quickly followed by a fresh segmentation, 

 the resulting segments being the rudiments of the permanent 

 vertebrae. The new seermentation, however, does not follow 

 the lines of the earlier division, but passes between the 

 ganglionic and the vertebral portions, in fact, through the 

 middle, of each protovertebra. In consequence, each spinal 

 ganglion and nerve ceases to form the front portion of the 

 primary vertebra, formed out of same protovertebra as itself, 

 but is attached to the hind part of the permanent vertebra 



