324 T. H. BAST 



is not different from that of the non-dividing nucleus as shown in 

 figure 1. Figure 4 shows the nucleus completely divided. The 

 two halves have moved in opposite directions and the cj^toplasm 

 at the plane of nuclear division is so constricted that the resem- 

 blance to true cell division is complete except for a narrow cyto- 

 plasmic connection. Whether such cytoplasmic connections 

 always remain as narrow canalicular processes or sometimes 

 divide completely to form two separate cells, I am unable to say. 

 Certain it is that some cytoplasmic processes which are continu- 

 ous with similar processes of sister cells are completely pinched 

 off in the process of growth and become separated by bone ma- 

 trix, but whether all of the processes are ever thus separated to 

 form non-communicating cells, I am unable to say, since I have 

 observed no such cases. 



Another important structure, the centrosome, should be noted 

 in connection with the dumb-bell-shaped nucleus. Centrosomes' 

 are not often clearly seen in dividing bone cells. This is espe- 

 cially true for this type of amitotic division because it is not 

 frequently found. Figure 2 shows a cell in which the centro- 

 somes are unmistakably distinct. They are placed in the cyto- 

 plasm at the two ends of the dumb-bell-shaped nucleus. The 

 centriole within one centrosome is somewhat elongated, but the 

 entire length was not visible in a single plane of focus. It 

 appeared to lie at an angle of about 45° with this plane. Fig- 

 ure 2 shows the centriole as it appeared under a shifting focus. 

 While in cells of this type centrosomes are not commonly seen, 

 the centrosomes of this particular cell were so unmistakable, 

 and duplicated so perfectly other less distinct observations, 

 that it warrants the conclusion that this position of the centro- 

 some is the rule for the dumb-bell type of amitosis. 



By far the majority of the cells divide by the horseshoe type 

 of amitosis. In young bone almost any stage of this type of cell 

 division can be found, and one may often find every cell in some 

 stage of division. It usually happens, however, that in a given 

 field or even in a given parietal bone all dividing cells have 

 approximately the same nuclear form. The various stages in 

 the horseshoe type of amitosis are shown in figures 4 to 12. 



