THE HISTOGENESIS OF THE NERVOUS TISSUES 



311 



zone, non-cellular, into which nerve fibers grow. The ependymal zone contributes 

 cells for the development of the mantle layer (Fig. 298 D.) The cellular mantle 

 layer forms the gray substance of the central nervous system, while the fibrous 

 marginal layer constitutes the white substance of the spinal cord. 





mli 



mle 



D 



Fig. 298. — Three stages in the early development of the neural tube showing the origin of the syn- 

 cytial framework: A, from rabbit before the closure of neural tube; B, from 5 mm. pig after closure of 

 tube; C, from a 7 mm. embryo; D, from a ro mm. pig embryo, a, ependymal layer; b, boundary 

 between nuclear layer and marginal layer; j>, germinal cell; m, marginal layer; mle, mli, external and 

 internal limiting membranes; r, mantle or nuclear layer; p, mesoderm. 



The primitive germinal cells of the neural tube divide by mitosis and give 

 rise to the ependymal cells of the ependymal zone and to indifferent cells of the 

 mantle layer. From these arise spongioblasts and neuroblasts (Fig. 299 ). The 

 spongioblasts are transformed into neuroglia cells and fibers, which form the sup- 



