NORMAL HISTOLOGY. 



FIG. 107. Portion of spinal cord of human embryo, showing 

 velopment of ventral root-axones as outgrowths from ventral 

 (After His.) 



developi 

 neuroblasts. 



of ectodermic origin, and enclose the young developing nerve. After a nerve has be- 

 COOK enlarged by the distal ingrowth of fibrils, the sheath-cells wander from the periph- 

 ery among the fibrilte, and thus give rise to a network that divides the original fascicu- 

 lus into a number of secondary 

 bundles. The interfascicular 

 cells increase rapidly, the 

 subdivision continues, and 

 the bundles of fibrillae be- 

 come progressively smaller 

 and more compact until, 

 surrounded by membranous 

 septa, they become the a.ris- 

 cylinders of the individual 

 nerve-fibres, enclosed by the 

 neurilemma and its cells. 

 The endoneurium appears 

 comparatively late and, un- 

 like the neurilemma, is a prod- 

 uct of the mesoderm. Later 

 condensations of the meso- 

 dermic tissue around the defi- 

 nite bundles of nerve-fibres 

 and around the entire nerve- 

 trunk provide theperineurium 

 and the epineurium respect- 

 ively. The medullary sheath is acquired comparatively late, since it does not appear 

 until about the fourth month of foetal life, some tracts within the central nervous axis, 

 indeed, not obtaining the medullary coat until imm 



after birth. It is probable that formation of 

 the medullary substance is in some way influ- 

 enced by the axis-cylinder, resulting in the 

 deposit of the myelin droplets from the fluid 

 that surrounds the axial thread. Thus, the 

 axis-cylinder and the neurilemma are derived 

 from the ectoderm, while the medullary sheath 

 is produced through the influence of the axis- 

 cylinder, from the same source. 



The sensory ganglia develop from groups 

 of ectodermic cells that form a ridge, the gan- 

 glion-crest, on the margin of either lipof the 

 still open neural tube, just where the general 

 ectoderm passes into that lining the groove. 

 On closure of the latter, the ganglion-crests 

 fuse into a dorsal wedge-shaped mass that 

 becomes a centre of proliferation from which 

 cells migrate outwards over the dorso-lateral 

 wall of the tube. In consequence, a series of 

 segmentally arranged cell-aggregations ap- 

 pears on each side of the neural canal, these 

 collections being the rudiments of the later 

 spinal ganglia. Within them certain cells soon 

 become fusiform and, assuming the r61e of 

 neuroblasts, send out a process from each end. 

 One process, the axone, grows centrally, while 

 the other, the dendrite, extends peripherally 

 and becomes the chief part of a sensory neurone, the afferent nerve-fibre. 



dr 



dz 



dr 



FIG. 108. Cross-section of part of dorsal 

 region of human embryo, showing develop- 

 ment of spinal ganglion'; d z, v z, m z, dorsal, 

 ventral and marginal zones of early spinal 

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

 spinal nerve (sn); sg, spinal ganglion on dor- 

 sal root. X 75. 



The 



subsequent growth of the neurone is not symmetrical, but to one side, so that the two 

 processes approximate and, finally, join the cell-body by a common stalk, the neurone 

 being thus converted into an unipolar ganglion-cell. The centrally directed processes, 



