198 



HISTOLOGY 



But few stains will properly differentiate these fibrils from the cyto- 

 plasm of the cells and from other kinds of fibers. Figure 176 shows 



this separation effected by means of 

 the "Mallory " stain. 



These cells must not be con- 

 founded with the real connective- 

 tissue cells of mesodermal origin that 

 are found in the nerve tissue. Such 

 cells are especially prominent near 

 the blood vessels and can easily be 

 distinguished from neuroglia in many 

 ways. Figure 176 shows a compari- 

 son between the two kinds. Note 

 the finer and more numerous as well 

 as more lightly stained fibrils of the 

 connective tissue; also the nucleus, 

 which is far smaller in the neurog- 

 lia cells. 



In the invertebrate animals are 

 found all stages of specialization of 

 the neuroglia cell from the embryonic 

 neuroblast. The fully adult forms, 

 which will alone be shown here, usu- 

 ally have some portion of their body 

 on the surface of the nerve center 

 or nerve cord in which they lie, thus 

 indicating their origin from the embryonic surface of the body, of which 

 most of the nerve structures were formerly a part. Figure 177 shows a 

 portion of a nerve cord of a gasteropod mollusk, Sycotypus, cut in trans- 

 verse section. The neuroglia cells, here ependymal in form, are situ- 

 ated with their principal cytoplasmic masses, containing the nucleus, 

 on the periphery, and the branching cytoplasm, containing the fibrils, 

 extends toward the center of the nerve cord and branches freely to form 

 a support for the numerous bundles of nerve fibers running through it. 



A more involved form, of great specialization, is to be seen in the enor- 

 mous neuroglia cells found scattered in some of the nerve cords of the 

 common leech, Hirudo medicinalis. This huge cell is placed in the center 

 of the nerve cord and sends branching processes outward to the surface. 

 The neuroglia fibrils are contained in these processes, which produce and 

 maintain them. The processes, here as before, act as a supporting scaf- 

 fold for the nerve fibers that pass through the cord. They also flatten 

 against the surface of the cord to which they are fastened (Fig. 178). 

 Although these large cells are more than probably derived from the 



FIG. 177. Neuroglia cells and their 

 branches in a nerve cord of the gasteropod 

 mollusk Sycotypus. neu.c., neuroglia cells. 



