THE NERVOUS SYSTEM. 



481 



marginal layer. The increase in the thickness and circumference of the walls 

 of the tube and the resulting tensions may be a factor in this arrangement 

 cf the protoplasmic filaments. At the boundary between the marginal and 

 nuclear layers the reticulum appears to be especially dense. 



With the further increase and development of the nervous elements (see 

 p. 485) the radial arrangement of the spongioblasts noted above becomes more 

 and more obliterated. As shown by Golgi preparations, in their migration from 

 the lumen (Fig. 422) the spongioblasts lose their connection with the lumen, 



mil 



mv 



FIG. 421. Hardesty. Combination drawing from sections of pig of 15 mm. The upper part is 

 from a section of the same stage as the lower but stained by the Golgi method. By migra- 

 tion and differentiation the mantle layer has been formed. The cells remaining near the 

 lumen form the ependyma layer (ep.). b, Boundary between mantle and marginal layers; 

 ep, ependyma; mli and mle, internal and external limiting membranes; mv, differently 

 arranged mid-ventral portion of the marginal layer; r, radial filaments; cs, connective tissue 

 syncytium. 



their peripheral processes become abbreviated and disappear, and they finally 

 differentiate into the irregular branching neuroglia cells (Fig. 423). According 

 to Hardesty, there is simply a general nucleated mass which changes form 

 pari passu with changes in the enclosed differentiating nervous elements, 

 finally assuming shapes dependent upon the character of the spaces between 

 the formed nervous elements. An exception to this is a layer of nucleated 

 elements which remain next the lumen and form the ependyma cells which still 



FIG. 419. Pig of 7 mm., unflexed. Segment from the ventro-lateral wall of the neural tube; 



g, Germinal cells; mli, internal limiting membrane; mle, external limiting membrane 



r, radial, axial filaments of the syncytial protoplasm; p, beginning of pia mater. Hardesty. 

 FIG. 420. Pig of 10 mm., "crown-rump" measurement. Segment from lateral wall of neural tube. 



b, boundary between nuclear layer and marginal layer (m). Other references same as 



in 419. Hardesty. 

 a indicates the zone in which the dividing cells are located. Later, it is composed of the inner ends 



of the ependyma cells (column layer of His). 



