46 NERVOUS SYSTEM OF VERTEBRATES. 



them bordering upon the lumen or upon the external surface. 

 The columnar cells retain their position throughout Hfe, extending 

 from the central canal to the surface. Since they give rise to a 

 sort of meshwork or treUis which serves largely to support the 

 nervous elements, these cells are called spongioblasts. The inner 

 ends of neighboring spongioblasts fuse to form an internal hmiting 

 membrane bounding the central canal and the outer ends may 

 form a similar external limiting membrane. The nuclei of the 

 spongioblasts are situated in the middle and internal portions 

 of the wall, the outer portion remaining relatively free from nuclei. 

 Those spongioblasts whose nuclei occupy the internal ends of 

 the cells become the ependyma cells of the adult brain. 



Among the spongioblasts are cubical or rounded cells which are 

 multiplying rapidly, so that many of them contain mitotic figures. 

 They are known as germinal cells. While a few of these cells lie 

 in the thickness of the wall or even upon the external surface, 

 the great majority border on the central canal (Figs. 13 B, 14, 15, 

 16). The germinal cells are believed to give rise to all the nervous 

 elements in the brain and spinal cord and to the stellate cells 

 which with the spongioblasts make up the neuroglia. In general 

 the histor}' of both nerve cells and gha cells is the same. The 

 cells produced by the division of the germinal cells migrate or 

 are pushed toward the periphery of the cord or brain and then, 

 supported by the meshwork of the spongioblasts, undergo develop- 

 ment into specific nerve or glia cells. 



Those cells which are destined to form nerve cells are called 

 neuroblasts. These continue to multiply rapidly by mitosis but 

 the number of mitotic figures seen in the nervous system becomes 

 rapidly less in later stages of development and in the adult the 

 division of nerve cells is comparatively rare. The first step in 

 the formation of a functional nerve cell from a neuroblast is the 

 production of processes somewhat similar to the pseudopodia 

 of an amoeba. First the neuroblast becomes drawn out at one 

 end into a slender filament which is recognized as a nerve fiber 

 (Fig. 16). The presence of this cell fiber is the only mark by which 

 the neuroblast can be distinguished with certainty from spongi- 

 oblasts or glia cells. The nerv-e fibers may collect into bundles 



