AMITOSIS IN BONE CELLS 323 



ing cells are more abundant in the younger than in the older area, 

 yet in both regions the proportion is quite variable. This varia- 

 bility is also obvious in very young bone, although in it most 

 cells are in some stage of proliferation (figs. 10 and 12). (See 

 also figs. 2 and 6 of my previous article on bone cells.) Appar- 

 ently this dividing process is a periodic affair, as shown by the 

 fact that in a given region, especially in young bone, all the cells 

 are practically in the same stage of proliferation. Because of this 

 variability, the exact ratio of dividing to non-dividing cells can- 

 not be given. It may be stated, however, that in young bone 

 such as is found in one- to twenty-day-old rats, proliferating cells 

 are so abundant that almost any stage of division can be found 

 without difficulty. In older bone this occurs with less frequency. 



Nature of cell division 



In my previous paper it was shown that all cell division in 

 bone is accomplished by amitosis. This requires little further 

 description but can readily be noted in the accompanying figures 

 (pi. 1). There is no sign of chromatin rearrangement nor of the 

 disappearance of the nuclear wall in any case. At any stage the 

 chromatin of the nucleus is grouped in small masses and irregu- 

 larly distributed with a slight condensation at the periphery. 

 This structure should be compared with the nucleus of a non- 

 dividing cell .as shown in figure 1. 



Types of amitosis 



It is a very striking phenomenon that among dividing bone 

 cells two nuclear forms occur. In the one case the nucleus is 

 dumb-bell shaped, while in the other it . is horseshoe-like in 

 form. Amitotic cells with the dumb-bell nuclei are not very 

 common and usually occur in old bone, but may occasionally be 

 seen in j^oung bone also. Three stages of dumb-bell amitosis 

 are shown in figures 2, 3, and 4. In figures 2 and 3 the two ends 

 of the nucleus seem to be drawn in opposite directions, causing 

 a rarefaction of the nucleoplasm in the middle which is followed 

 by a constriction. The arrangement of the nuclear chromatin 



