RAND: NERVOUS SYSTEM OF LUMBRICID.E. 135 



8. The Centrosome in the Mature Kegenerated Nerve 



Cells. 



Evidences of the preseuce of the centred system are to be found in 

 cells of all sizes from those represented in Figures 40—42 (Plate 6) to 

 the fully differentiated nerve cells such as are shown in Figures 10-13 

 (Plate 2), which are from a brain of thirty-four days' regeneration. 

 The cells shown in Figures 10-12 lay side by side in a single section of 

 the brain. Figure 10 represents a cell whose centred system places it 

 among cells of the "first type," as described (pages 112-114) for cells 

 of the normal animal. The whole structure in this case was unusually 

 distinct. The centrosome (for so it can now be called) lies in the axis 

 of the cell and so close to the nucleus that the small clear space about 

 the centrosome is tangent to the nuclear membrane. Four distinct 

 radiations extend nearly or quite to the periphery of the cell. They 

 appear finely granular. The presence upon the radiations of conspicu- 

 ous granules surrounded by small clear spaces can be seen. The two 

 radiations nearest the axis of the cell appear connected by a cross 

 fibre, which extends between two of the larger gi-anules mentioned. 



In Figure 11 there is a distinct centred system with its centrosome 

 in the axis of the cell and near the nucleus. About the centrosome 

 is an imperfectly described circle of granules, four of which lie on the 

 paths of radiations from the centre. In this respect, the system is one 

 of the "second type" (page 114), but the region bounded by the circle 

 of granules is rather lighter than the surrounding cytoplasm, instead of 

 denser, as in Figure 3. A small neuroglia nucleus lies close upon one 

 side of the cell. 



Figure 12 shows a clear circle tangent to the nucleus and in the 

 cell axis. At the centre of the clear circle are three indistinctly 

 defined granules, from which several faint radiations extend into the 

 cytoplasm. 



Figure 13 exhibits a remarkable complexity of the centred system. 

 The cytoplasm in this case was practically unstained, yet its fibres 

 stood out with much clearness. This cell shows to advantage the presence 

 of secondary centres. The primary centre, or centrosome, is easily distin- 

 guished as lying in the cell axis and in being the point from which pro- 

 ceed several radiations which extend in straight lines almost to the 

 periphery of the cell. Upon these radiations are conspicuous granules 

 from which extend secondary radiations. The secondary radiations some- 



