238 The Development of the Neuroglia 



and assumes the form shown in Fig. 4. Secondarily, both the nucleated 

 layer and the mantle layer thicken in the dorsal portion also. As the 

 wall of the tube increases in thickness, the syncytial protoplasm, being 

 continuous throughout and common with both the internal and exter- 

 nal limiting membranes, suffers a further spinning out. In other words, 

 the axial threads, though gaining in the actual amount of their sub- 

 stance, are drawn out to greater individuality and more complete radial 

 arrangement. In the radial direction of the growth, the coarse mesh- 

 work of the mantle layer, as shown in Fig. 4, is so drawn out that its 

 meshes become radially elongated. In the process of elongation the 

 filaments of many of the smaller meshes become adjacent and fuse 

 with the larger threads, thus obliterating the smaller meshes and 

 thickening the threads perpendicular to the limiting membrane. In 

 this way the radial arrangement of the mantle layer is accentuated by 

 a mechanical addition to certain of its threads, the smaller lateral 

 threads connecting them appearing less significant by contrast, and 

 the apparent bifurcation of the radial axes of the nucleated layer be- 

 comes more pronounced. 



The general radial arrangement of the syncytium thus produced is 

 maintained up to pigs of 7 centimeters, when it is destroyed in a way 

 to be described later. In pigs of 3 to 5 centimeters the memhrana 

 limitans externa becomes less distinct. It either decreases from being 

 spun out into the general framework, or the syncytium of the mantle 

 layer so increases in density as to destroy the contrast. 

 . In the literature the radial axes, etc., are almost exclusively de- 

 scribed as cell-processes, and the nuclei nearest the ventricle (ependy- 

 mal cells) are given all the credit for the entire framework. Hannover, 

 44, and Stilling, 59, were, I think, among the first to describe them 

 as such. Though primarily dealing with the development of the nerv- 

 ous elements, several of His' papers touch upon the subject. Of neces- 

 sity he considered the early, coarse reticulum of the mantle layer as 

 produced by the anastomosis of greatly extended and much branched 

 peripheral outgrowths of the ependymal cells. Among the others, 

 Weigert accepts this form of description and, quoting Sala y Pons, 

 states that the ependymal cells send out two processes, one to the ven- 

 tricle and one to the periphery. From the method of its origin, the 

 nature of its structure, and from its later modifications, I do not 

 think the term cell-processes adequately describes the framework. 



Many of the investigators have based their conclusions solely upon 

 appearances obtained with the silver method. In the first place, the 

 silver salt will not differentiate the earliest stages, and in the second 



