THE NERVOUS SYSTEM. 479 



Epithelial Stage. Development of Neuroglia. 



From the very first, the neural plate exhibits dividing cells similar to those 

 seen in the non-neural ectoderm. The cell divisions are indirect and the 

 mitoses are confined to the outer part of the ectoderm, occurring between the 

 outer ends of the resting epithelial cells (Fig. 417). These dividing cells have 

 been termed by His germinal cells. When the neural tube is formed, the 

 mitoses are still confined to the outer, now the luminal, surface, this being a 

 general phenomenon in developing epithelial tubular structures. As a result 

 the daughter nuclei migrate away from the lumen. 



In the most advanced parts of the neural tube (see p. 478), the mitoses in- 

 crease in number up to about the fourth to sixth week of development, and then 

 diminish and finally nearly disappear about at the end of two months. At 

 about the time the blood vessels penetrate the tube, the mitoses .are no longer 

 entirely confined to the proximity of the lumen. 



As a result of proliferation, the epithelial wall very early assumes the ap- 

 pearance of a stratified epithelium at least there are several strata of nuclei. 

 There are at this stage in many forms two layers, an outer or marginal layer, 

 free of nuclei, and an inner or nuclear layer (Figs. 418 and 419). In a human 

 embryo, however, of about two weeks this division into layers is yet hardly 

 evident, though there are several strata of nuclei. Apparently these layers are 

 not well-marked until the radial arrangement of the myelospongium, as 

 described below, has become more pronounced. 



Accompanying the above changes, changes also manifest themselves in the 

 character of the cells. At about the time of the closure of the neural tube, the 

 cell boundaries become indistinct and finally practically obliterated, thus form- 

 ing a syncytium, the myelospongium. At the same time, the syncytium becomes 

 very alveolar in structure and a general spongioplasmic reticulum is formed (Figs. 

 418 and 419) by the anastomosing denser strands (trabeculae) of protoplasm. 

 At a very early stage (two weeks), these trabeculae unite along the inner and 

 outer walls of the neural tube forming internal and external limiting mem- 

 branes. The nuclei of the neural tube have at first an irregular arrangement 

 in the reticulum, at least in the human embryo. This is followed by a more 

 radial arrangement of both nuclei and protoplasmic filaments (Fig. 420) , form- 

 ing nucleated radial masses of protoplasm the sponglioblasts (Figs. 419 to 

 422). There is some dispute as to the loss, complete or incomplete, of identity 

 of the epithelial cells in the formation of the spongioblasts. According to 

 Hardesty, they are formed by a collapse of the epithelial cells and a rearrange- 

 ment of their denser parts into axial filaments. The radial arrangement does 

 not extend into the outer part of the neural tube which, retaining its irregular 

 reticular character, is now non-nucleated in the human embryo and forms the 



