578 SUMMARY OF CURRENT RESEARCHES RELATING TO 



indissolubly by intermediate forms, are distinguishable: — (1) cells 

 in which the nucleus is relatively very large and often indeed 

 apparently naked, at least always forming the chief part of the 

 cell ; from the nucleus itself or from the little protoplasm round 

 it, a few processes arise, which are always extremely delicate and 

 branching ; these cells are most abundant in the grey matter. 

 (2) cells in which the nucleus is cither absent, stunted, or ill- 

 defined, but with well-developed, and in adults well-cornified cell- 

 bodies, with firm and numerous processes. The differences between 

 these two forms are described in great detail. 



Cornification. — Neurokeratin was described by Ewald and Kiihne 

 in 1877; Gierke has shown the exact mode of its origin from the 

 gradual cornification of the cells and their processes. In its study 

 use was made of the digestive method (with pepsin and trypsin), 

 whose application is due to the above-named investigators. While 

 the keratin-forming process advances the nucleus degenerates, gets 

 smaller and crumpled, and finally disappears in the cell. The 

 further this degeneration of the nucleus has advanced, the greater the 

 resistance of the cell to acids and alkalies. In the second form of cell, 

 where the nucleus predominates, the keratin-modification is confined to 

 the processes ; neither the nucleus nor the small cell-body are cornified. 

 Development. — In the embryonic nerve-strand the cells are of 

 course alike, nor after the nerve-fibres are differentiated are there 

 any observable differences in the roundish polygonal " Stiitzzellen," 

 and even when the glia-cells have developed their characteristic pro- 

 cesses they are for a while quite homogeneous. Glia-cells and nerve- 

 cells have emphatically an identical ectodermic origin ; the former 

 certainly do not come in from outside with the blood-vessels or in 

 any other way. Gierke protests emphatically, as we have noted, 

 against the common use of the phrase "connective tissue of the 

 central nervous system." The neuroglia is entirely ectodermic, 

 connective tissue is mesodermic, and the histological structure of the 

 two is very different. 



He describes in greater detail the history of the " epithelial glia- 

 cells " which limit the glia round the central cavities of the brain 

 and spinal cord, noting especially the latter. At first several layers 

 of long elliptical or spindle-shaped cells are seen regularly arranged 

 round the central canal. The peripheral ends of these cells are 

 narrowed and prolonged in a process in the direction of the longi- 

 tudinal axis of the cell, and can be followed through the grey and 

 white matter to the margin of the cord. The outer cells of these 

 layers become much modified from without inwards ; from them 

 originate both ganglion-cells and multipolar neuroglia-cells ; a single 

 layer is at length left — the epithelium of the central canal. Processes 

 from the lateral " epithelial cells " join the fibres of the glia mesh- 

 work of the substantiva gelatinosa centralis, or penetrating through 

 this unite with the fibres in the grey substance. The epithelial cells 

 before and behind the canal send sagittal processes anteriorly and 

 posteriorly between the symmetrical halves of the cord, uniting with 

 the connective-tissue sheath. Independent fibrils may arise by losing 



