140 BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY. 



single, very dark, spherical granule lying at the pole of the spindle. At 

 the close of mitosis it increases considerably in size, and sometimes ap- 

 pears less intensely stained. In Figures" 31-35 (Plate 5) the centrosome 

 is seen to be much larger than at any stage of the mitotic figure. In 

 Figure 34 it is not a spherical granule, but is somewhat elongated. In 

 cells showing a greater development of the cytoplasm (Figures 36-38) 

 there is generally present a large dark granule near the centre of the cyto- 

 plasmic mass. In as early conditions as those seen in Figures 32 b, 35, 

 37, and 38, the differentiation of a clear space, or a darker circle, about 

 the centrosome, or some evidence of radiations can sometimes be seen. 

 The centrosome is largest immediately after mitosis. As the cytoplasm 

 increases in volume the centrosome becomes somewhat smaller. In cells 

 like those of Figure 39, where the nucleus is in typical resting condition, 

 the small cytoplasmic mass contains the centrosome, frequently with 

 evidences of concentric and radiating structures about it. With further 

 increase in the size of the cell and the volume of the cytoplasm (Figures 

 40-42) there is to be seen a centred system — centrosome, clear space 

 and radiations — comparable in all respects to the system seen in the 

 mature nerve cell. 



In all stages of development between the last mitosis and the mature 

 cell, the centrosome and its accompanying structures are so frequently 

 met with that one is warranted in concluding that they are generally 

 present. 



It is evident, then, that the centrosome of mitosis is present during 

 the earliest stages in the reconstitution of the daughter cells, that it 

 persists during the growth of the cell, becoming associated with certain 

 concentric structures and radiations, and becomes finally the central 

 body of the complex system of radiating and intersecting fibres found in 

 the fully differentiated nerve cell. This is true for the regenerated cells. 

 That it will be found true in the case of nerve cells developed by the 

 normal embryonic process is hardly to be doubted. 



4. The Centrosome in Nerve Cells. 



In transverse sections of a ganglion the cells are most likely to 

 lie with their axes nearly parallel to the plane of cutting. In such 

 sections evidences of the centred system are often to be found in the 

 majority of the pear-shaped nerve cells. Thus the three cells of Figures 

 9-11 (Plate 2), from a regenerated brain, lay side by side in the same 

 section. 



