DEVELOPMENT OF THE NERVOUS TISSUES. 



1013 



Fig. 861. 









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r,'t^<:^V.'Av-^-^" 



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Cross-section of part of dorsal region of human 

 embryo, showing developing spinal ganglion ; dz, 

 vz, VIZ, dorsal, ventral and marginal zones of 

 spinal cord ; dr, vr, dorsal and ventral root-fibres 

 of spinal nerve («) ; sg, spinal ganglion on dorsal 

 root. X 85. 



ordered that the two processes are approximated and finally joined to the cell-body by a 



common stalk (Fig. 839), the neurone being thus converted into an unipolar ganglion-cell. 



The centrally directed processes, the later posterior 



root-fibres of a spinal nerve, grow into the develop- 

 ing cord and enter the peripheral zone (later the 



white matter) to end, when their development is 



completed, at various levels in relation with neu- 

 rones formed within the neural axis. The peri- 

 pherally directed processes of the spinal sensory 



neurones, on the other hand, mingle with the 



axones from the motor neurones to form the mixed 



nerves distributed to the various parts of the body. 



The essential parts of the sensory neurones, the 



cell-body and the processes, are derived from 



ectoblastic elements, as well as the sheaths of the 



fibres, while the sheath of the entire ganglion is 



contributed by the mesoblast. 



The development of the sympathetic ganglia, 



which include essentially three sets — those of the 



gangliated cords, those of the prevertebral plexuses 



(cardiac, solar and hypogastric), and the terminal 



— has given rise to much discussion. According 



to one view, the sympathetic neurones have an 



independent origin and only secondarily form con- 

 nections with the cerebro-spinal nerves. The other 



view, on the contrary, regards the sympathetic 



neurones as the direct descendents of neurogenetic 



elements derived from the developing spinal nerves. 



The evidence in support of the last view is so 



convincing that there is little question as to the 



correctness of its principle, although many details 



of the process, as relating to man, are still to 



be studied. It is, however, equally true that the 



sympathetic ganglia are neither produced by constriction and isolation of parts of the spinal 



ganglia, as sometimes assumed, 



Fig. 862. . . nor by the migration of fully 



..'•_•.; . .^ differentiated ganglion - cells, 



but, as emphasized by Neu- 

 mayer, from undifferentiated 

 neuroblasts which undergo in 

 loco their development. The 

 earliest suggestions of definite 

 sympathetic ganglia in the 

 human embryo appear about 

 the beginning of the second 

 foetal month as aggregations of 

 cells at the distal ends of the 

 visceral rami of the developing 

 spinal nerves. From these cells 

 are derived the definite sympa- 

 thetic neurones of the gangliated 

 cord, as well as those which 

 follow the mesial ingrowth of 

 the spinal fibres for the pro- 

 duction of the prevertebral and 

 terminal ganglia. The lateral 

 ganglia thus formed constitute 

 for a time a series of isolated 

 nodes ; subsequently these are 

 connected by the differentiation 

 of sympathetic axones which 

 grow from one ganglion to the 

 next and, in conjunction with the spinal fibres, establish the longitudinal commissural strands 

 of the gangliated cord. Other sympathetic cells send axones centrally and give rise to the 

 efferent splanchnic nerves, whilst the axones of still others pass to the growing spinal nerves. 



Nerve- 

 trunk 



Spin 

 gangl 



Sagittal section of rabbit embryo showing several developing spinal ganglia 

 and nerve-trunks; A, aorta; S, intersegmental artery. X 52. 



