Irving Hardesty M9 



fibrous tissue in tlie central nervous system give the neuroglia reaction, 

 by this reaction they would be classed as neuroglia fibers. 



The Final Form of tpie Syncytium and the Development of the 

 Neuroglia Fibers from it. 



The syncytium is formed early, before the embryonic nervous system 

 is invaded by ingrowths of mesodermal tissue, and thereafter is manifest 

 in all stages. Its arrangement, however, changes as the specimen grows 

 and acquires the form and structural components of the adult. Its 

 substance being plastic, its variations are expressive of the processes of 

 growth. At first, resulting from the fusion of radially arranged colum- 

 nar cells, it soon assumes the form of a sort of filamentous reticulum 

 continuous with the internal and external limiting membranes. Then, 

 as the nuclei of the inner zone proliferate and migrate along radial lines, 

 and as the external limiting membrane grows further away from the 

 internal in the thickening of the wall of the tube, the syncytium assumes 

 the form of radially arranged thicker filaments intimately continuous 

 with each other by the more attenuated portion of the reticulum between 

 them. As the specimen grows further, the radial filaments thicken and 

 are further drawn out, and for a time, the radial arrangement becomes 

 more marked. It is finally obliterated by the ingrowths and medulla- 

 tion of the neuraxes and the further structural changes toward the adult 

 form. Then the syncytium, by its plasticity, assumes the shapes of the 

 interspaces of the elements Avhich it supports. 



The " neuroglia nuclei " begin to show variations in pig embryos of 

 20 millimeters. Previous to this they are all of the large vesicular type. 

 At 30 millimeters (Figs. 8 and 9), while the majority of the nuclei are 

 still of the large vesicular variety, many may be seen undergoing changes 

 which in all probability result in the various forms usually described 

 in the adult tissue. The changes consist in a decrease in size and a more 

 compact arrangement of the chromatin resulting in deeper staining. 

 The smallest appear as blue-black spheres of less than half the diameter 

 of the large vesicular form. At 30 millimeters, when the migration has 

 resulted in the demarcation of the dorsal horn, the small nuclei are more 

 abundant in the dorsal horn than elsewhere in the section. 



ITp to about 25 millimeters the ventricle increases in size; then, with 

 the cessation of mitosis and the thinning of the ependymal layer, it 

 decreases in size by a collapse of its dorsal two-thirds and a fusion of the 

 internal limiting membrane along the mid-line (compare d, Fig. 6, with 

 sp. Fig. 8). From 30 millimeters the ventricle continues to decrease, 



