IOIO 



HI 'MAN ANATOMY. 



been sustained ( Kollike: and othersi since the primary germinal cells probably only 



represent proliferating elemen. in forming wliat for a time is an undinerentiated tissue. 



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



ytiumj forming an almost unbroken sheet. Soon, however, this continuity is interrupted 



in ( onsequence of the longitudinal expansion of the tissue and the appearance of spaces, and the 



cell-substance is resolved into a delicate reticulum, the m\fh>sf>oHginm of His, which becomes 



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



ml limiting monln tint: 



The ni'-shes of the reticulum enlarge, the intervening nucleated tracts of cytoplasm elongate 

 and the increasing nuclei become radially disposed. My reason uf these changes the elements 

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

 primary t-f>cndymul idls. The remaining elements, appropriately named the indifferent cells 

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

 gradually - the nut 'It'ar /<mr 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 



,r g iim 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 (f>) the neuroblasts 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 difficult 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<ells and fibres) that the primary syncytial meshwork undergoes 



FIG. 857. 



Segment of wall of neural tub* of pig embryo of 10 mm. ; radial 

 . :!iim ami differentiation of ependymal (a), nuclear 

 (6) and marginal (m) layers; ilm. flm, internal and external limiting 

 membrane; jr, dividing ceil ; /, pia mater. X 690. (Hardf.: 



Tran'--. ord from pig embryo of v aim upper rwrt of 



ncilxvilli tiiliiiilin Mm-; (.mitral raiial 'ef ependvnnl 

 ll plati- uniting halves of cord v it, 



I llar.i, 



(Hardestv), lie- gradual transformation of the spongioblasts and their descendants into fibrillse 

 ;sli,-s a more definite framework that replaces the primary net-work i mvelospoiigjum), and 

 eventually, in COIIJUIK lion with the fibril!. e derived from the processes of the ependymal cells 



