lOIO 



HUMAN ANATOMY. 



been sustained (Kolliker, Schaper and others) since the primary germinal cells probably only 

 represent proliferating elements engaged in forming what for a time is an undifferentiated tissue. 

 The cells composing the neural wall are at first in close contact, their blended cytoplasm 

 (syncytium) forming an almost unbroken sheet. Soon, however, this continuity is interrupted 

 m consequence of the lorf^itudinal e.xpansion of the tissue and the appearance of spaces, and the 

 cell-substance is resolved into a delicate reticulum, the myelospongium of His, which becomes 

 condensed at the inner and outer margins of the wall of the neural tube into the internal and 

 external liiniting membrane. 



The meshes of the reticulum enlarge, the intervening nucleated tracts of cytoplasm elongate 

 and the increasing nuclei become radially disposed. By reason of these changes the elements 

 next the lumen of the tube assume a columnar form and radial arrangement and become the 

 primary epetidynial cells. The remaining elements, appropriately named the indifferent cells 

 (Schaper), increase in number in consequence of the continued division of the germinal cells and 

 gradually become collected as the nuclear layer at some distance beyond the ependymal zone. 

 Meanwhile and very early, the peripheral portion of the supporting framework adjoining 

 the outer border of the neural wall becomes denser and free from nuclei and is converted into 



the marginal zone ( Randschleier of 

 Fig. 856. His), that is continuous with the 



delicate reticulum pervading the 

 other parts of the wall. The in- 

 different cells later differentiate 

 into (a) the spongioblasts from 

 which the characteristic constitu- 

 ents of the definite supporting 

 tissue, the neuroglia, are derived, 

 and {b) the ?ietiroblasts that are 

 directly converted into the neu- 

 rones. Within the resulting cell- 

 complex that for a time occupies 

 the greater part of the wall of 

 the neural tube, it is difificult to 

 distinguish with certainty between the .neuroglia and neuron-producing elements, since both 

 are often elongated in shape and prolonged into processes. 



Histogenesis of the Neuroglia. — In addition to the extension, condensation and moulding 

 (by the developing nerve-cells and fibres) that the primary syncytial meshwork undergoes 



elm 



ibn 



6!,, 



\ p m b a 



Segment of wall of neural tube of pig embryo of 10 mm.; radial 

 strands {r) of syncytium and differentiation of ependymal (a), nuclear 

 {b) and marginal (m) layers; //»/, ."/w, internal and externallimiting 

 membrane; g, dividing cell ; /, pia mater. ,■ 690. (Hardesty.) 



Fig. S57. 



^lil 





/\ 



L^ 



Transverse section of ventro-lateral segment of developing spinal cord from pig embryo of 30 mm., upper part of 

 figure from chrome-silver preparation, lower part from one stained with toluidin blue: r, central canal; .»/>, ependymal 

 layer; «, nuclear layer; »j, marginal layer; r, radial fibres; v, ventral plate uniting halves of cord. X 240. 

 (Hardesty.) 



(Hardesty), the gradual transformation of the spongioblasts and their descendants into fibrillae 

 establishes a more definite framework that replaces the primary net-work (myelospongium), and 

 eventually, in conjunction with the fibrillie derived from the processes of the ependymal cells, 



