98 



NERVOUS SYSTEM 



protoplasmic process grows to become the axis- cylinder process of the future nerve- 

 cell, and by further extension a nerve-fibre. The dendritic processes are secondary 

 outgrowths which, according to the manner of their branching, confer distinctive 

 characters on the several varieties of nerve-cells. The neuroblasts occupy the 

 spaces of the myelospongium, and the nerve-fibre processes thread its interstices. 

 The spongioblasts form supporting elements merely. According to another inter- 

 pretation, founded on the conception of nerve-tissue put forward by Apathy and 

 Bethe, and advocated by Held, 1 the neuroblasts are cells of the syncytium in which 

 neurofibrillae are differentiated. These form a plexus in the cell-body, surround 

 the nucleus and extend first, as a chief bundle (pear-shaped stage), through one of 

 the cell-bridges and along a definite path through the syncytial framework to form 

 the axis-cylinder process, and later into the other cell-bridges to form the dendrites. 

 While this interpretation differs from the first in so far as that the nerve-fibres are 

 regarded as neurofibrillar tracts in the substance of the myelospongium, not free 



processes threading its meshes, 

 the two theories agree in respect 

 that the nerve-fibre is represented 

 as an active outgrowth from 

 the nerve-cell. On this point 

 Held's view differs from that 

 of Apathy, who conceives the 

 conducting fibrillae as being laid 

 down in situ along a protoplasmic 

 path provided by cells (' nerve- 

 cells ') united into a syncytium. 



Origin of the nerve- 

 roots. The motor nerve-root* 

 appear before the sensory. In 

 mammalian embryos, when first 

 detected, the rudiments of the 

 motor nerves appear as bundles 

 of extremely delicate proto- 

 plasmic fibrillae extending among 

 the mesenchyme- cells from the- 

 wall of the neural tube, within 

 which the bundles can be traced 



backwards through the commencing reticular framework to neuroblasts in 

 the mantle zone (fig. 133). Followed outwards, they pass towards the myotome 

 (fig. 134). In the case of the spinal nerves each motor root is joined by a 

 contingent of fibrillae from the spinal ganglion, and the mixed nerve so forme 

 is seen extending into the Wolffian ridge along the mesial aspect of the growing 

 muscle -rudim ent . 



The sensory roots are developed from primitive ganglia which arise from 

 the neural crest. This is formed of cells derived from the ectoderm at the 

 angles of the neural folds (fig. 135). From the sides of the crest cells ai 

 budded off into the space between neural canal, myotome, and surface ectoderm, 

 and are there collected into paired groups, which are the rudiments of 

 the spinal and cranial nerve-ganglia. In the trunk these are segmentalb 

 disposed. It is possible that only a certain proportion of the cells 

 so utilised, for there is reason to believe that some of them wander farthei 

 afield. Each ganglion is at first connected with the side of the neural tube by 



FIG. 135. Two STAGES IN THE DEVELOPMENT OF THE 



NEURAL CREST IN THE HUMAN EMBRYO. (LenllOSSek.) 



1 Anat. Anzeiger, Erganzungsheft, xxix. 1906 ; and Anat. Anzeiger, xxx. 1907. 



