262 The Development of the ISTeuroglia 



have to do with the supporting structures of the myelin sheath, for this 

 sheath in the central as well as in the peripheral system possesses a 

 primitive sheath and a delicate framework throughout, contrilmting to 

 its organization. In the central system the primitive sheath is much 

 thinner than in the peripheral nerves, but it may be discerned in prepara- 

 tions from which the myelin has been extracted, and also the delicate 

 framework which permeates the myelin itself. Some of the features of 

 this framework have been recently described by Wj^nn, oo, and Hatai, 03, 

 who give the findings of previous observers, and my preparations show 

 that both the primitive sheath and the framework not only resist diges- 

 tion, as first shown by Ewald and Kiihne, 76, but also, after the special 

 neuroglia method, they both stain like the mesodermal tissue from the 

 pia mater. 



SUxMMARY. 



1. The cells composing the neural tube are at first individual and 

 definitely arranged, but at an early stage they all lose their boundaries 

 and the resulting fusion of their protoplasm gives rise to a syncytium. 



2. The protoplasm of the syncytium increases more rapidly than the 

 nuclei are distributed and, in consequence, there appears at the periphery 

 of the embryonic spinal cord an excrescence of non-nucleated protoplasin, 

 which becomes the mantle layer of the later stages. 



3. The fine threads of the spongioplasmic network of the original cell- 

 protoplasm thicken and the meshes enlarge, giving rise to a filamentous 

 reticulum, and, at the peripheral and ventricular surfaces of the neural 

 tube, this reticulum becomes condensed into the external and internal 

 limiting membranes. Thus the specimen becomes a reticulated syncy- 

 tium with definite boundaries. 



4. The threads further thicken both by growth of their substance and 

 by a condensation of adjacent threads, resulting from the collapse of 

 many of the smaller meshes occasioned by a radial drawing out of the 

 reticulum. This radial drawing out and condensation is due partly to 

 the original form of the syncytium, but largely to the lateral direction 

 of the growth of the wall of the tube and the radial migration of the 

 nuclei from the ependymal layer toward the periphery. It continues 

 till the syncytium of the lateral Avail assumes the form of radially 

 arranged, axial filaments connected with each other by numerous smaller 

 threads between them. Due to the nature of their formation, the axial 

 filaments apparently bifurcate near the inner boundary of the mantle 

 layer. Here the bifurcations, together with the more numerous lateral 

 threads, result in a complexity in the arrangement of the filaments which, 

 for a time, prevents the nuclei from migrating into the mantle layer. 



