Irving Hardesty 253 



indeed such are often isolated either by section or by incomplete precipi- 

 tation of the silver. In this way it is possible by the silver method to 

 get " neuroglia cells " without nuclei. For example, in Fig. 11 the 

 bodies designated by c in all probability contain nuclei, while those 

 indicated by b do not. When the radial arrangement of the syncytium 

 is broken up, these bodies, more abundant and more marked than in the 

 earlier stages, naturally become isolated in the process. A further 

 study of silver preparations of the later stages leads to the conclusion 

 that, while most of these bodies usually descril)ed as neuroglia cells do 

 contain nuclei, many of them do not. 



One of the first evidences of the breaking up of the radial arrange- 

 ment is the rupture and pulling away of the ends of certain of the axial 

 filaments from their attachments. This beginning is shown in Fig. 11 

 (pig of T centimeters) where at least two of the filaments with their 

 interposed nuclei (e) have lost their direct continuity with the internal 

 limiting membrane, and in the mantle layer several seem broken away 

 from the periphery. At the same time occasional nuclei (c), other than 

 those of the ependymal layer, begin to be selected by the silver and the 

 fine filaments immediately about them give the well-known figures. 



The radial filaments have been frecjuently described but usually as pro- 

 cesses of ependyma cells. Lenhossek. 95, thinks their breaking away 

 from both the central canal and from the periphery is the result of their 

 contraction. Leaving aside their manner of origin, if the ependymal 

 nuclei with their common protoplasm can be considered as cells, then for 

 a time the radial filaments, after the silver method, do resemble pro- 

 cesses, but processes continuous with each other by means of numerous 

 .smaller filaments between them. The rupture of the axial filaments, is, 

 I think, more probably due to unequal growth processes than to their 

 ■contraction. At the time the rupture occurs the wall of the neural tube 

 is rapidly thickening by the ingrowth of new nerve-elements, by the 

 enlargement of those already there, and by the increase and extension 

 •of the blood-vessels, and the resulting tension is probably greater than 

 the rate of growth of the filaments. 



With the further enlargement of the specimen, the ingrowth, arrange- 

 ment, and elaboration of the neurones and blood-vessels, the obliteration 

 of the radial arrangement continues, till, as is well known, a state is 

 reached in which no vestige of the radial arrangement remains save in 

 the then thin ependymal layer immediately surrounding the central 

 canal. Here such an arrangement is maintained even in the adult. 

 This arrangement broken up, the SAmcytium. by the ordinary staining 

 methods, is even more apparent than before and consists of a continuous, 

 19 



