AMITOSIS IN BONE CELLS 325 



Clear pictures of centrosomes are fairly common, but in this 

 connection it should be remembered that a very large number of this 

 type of cells are available from which to pick good stages. That 

 many of the cells do not show centrosomes clearly can be ex- 

 plained by the fact that they may be obscured by other bone cells 

 lying at a higher or lower level; or the cell may be so oriented 

 that its own nucleus is the obscuring factor; or again in many 

 cases protoplasmic processes viewed on end appear as deeply 

 stained dots easily confused with granules and often obscuring 

 the centrosome; or, finally, indistinct vacuoles lying in the 

 granular cytoplasm appear so much like centrosomes that the 

 picture is confusing. Thus in many cells the centrosomes are 

 present, but cannot be cited as typical because of these obscuring 

 structures. The various possibilities of the horseshoe type of 

 amitosis are shown in figures 5 to 12. Figure 5 is a typical 

 example of a cell in the early stages. At first the nucleus is 

 kidney shaped with the centrosomes in the position of the hilum. 

 The bending of the nucleus then increases so as to produce the 

 horseshoe shape. The centrosomes do not migrate apart as they 

 do in the dumb-bell-shaped nucleus, but remain in the concavity. 

 It seems that the nucleus changes in such a way that each part 

 of it remains as near the centrosome as possible. Often the two 

 ends of the nucleus encircle the centrosomes to such an extent 

 that they touch each other or even overlap. In this case, 

 when viewed from the side the nucleus is ring-shaped with the 

 centrosomes enclosed within. 



A dividing nucleus is shown in figure 6. Division here is 

 different from that in the dumb-bell type. Instead of becoming 

 elongated, the nucleus is pinched in two without any perceptible 

 moving apart of the daughter nuclei. The resulting kidney- 

 shaped nuclei then separate a little (fig. 7) and the cell engages 

 in cytoplasmic constriction (fig. 8). Later stages in cytoplasmic 

 constriction are seen in figures 10 and 12. 



Nuclear division does not always result in only two nuclei, 

 but nucleic horseshoes sometimes split into three, four, or more 

 daughter nuclei (figs. 9 and 11). Multinuclear cells . are not 

 numerous and are found in young bone. Whether such cells 



