kand: nekvous system of lumbricid^. 141 



A centred system is not to be made out, however, in every cell of a 

 section. Even if the structure is one generally present, we should not 

 expect to be able to see it in all cells, for only tlie most favorable condi- 

 tions could bring it to view. If the axis of the cell is not approximately 

 parallel to the plane of cutting, the cell is likely to be cut so that the 

 centrosome is not in the same section with the nucleus, — a condition 

 which increases the difficulty of identification. So delicate a structure 

 may often be obscured by some of the darkly staining masses of the 

 cytoplasm. Considering the difficulties of observation, then, the centred 

 system can be made out in so large a proportion of cells as to justify 

 the belief that it is a structure usually, if not always, present in the 

 mature nerve cells. 



The largest cells of the brain and subcBsophageal ganglion were least 

 satisfactory for showing the presence of this structure. It was found 

 with greatest frequency in cells of medium size, like those shown in 

 Figures 1-4. 



The " centrosomes " and '' spheres " described for nerve cells by va- 

 rious authors present such widely different conditions as to suggest 

 that they are not all homologous structures. For example, the condi- 

 tion in the centred system as I have found it in the earthworm is in 

 no way similar to the sharply outlined homogeneous sphere and its 

 numerous central granules described by von Lenhossek for spinal gan- 

 glion cells of the frog. The structure in such a cell as is represented in 

 my Figure 3 may suggest his '' sphere," yet it differs in being bouuded 

 by a layer of granules which occur on the radial fibres. Von Lenhossek 

 found no granules bounding his sphere, nor any radiations. 



The structure described by Dehler resembles that of von Lenhossek, 

 being equally unlike anything found in the earthworm. 



It is not impossible that the centrosome of mitosis, persisting in 

 the resting cell, may give rise to structures differing as widely as 

 the centrosome and sphere of von Lenhossek and Dehler differ from the 

 centred system which I have described. A fuller knowledge of the 

 structures described by them is needful. As long as they can be in- 

 terpreted as sections of a "spiral figure' (Holmgren), or as wandering 

 nucleoli (Eohde), a detailed comparison of these with the centred system 

 is hardly warranted. 



The methods used by Schaffer were not specially adapted to the study 

 of finer cytoplasmic structure. McClure examined Lumbricus and found 

 no evidence of the centrosome. The centrosome and s])here described by 

 him for the nerve cells of other invertebrates exhibited no radial struc- 



