322 T. H. BAST 



Staining with iron hematoxylin was attempted, but without suc- 

 cess. It was hoped that a clearer picture of the centrosomes 

 would be obtained by this method, but the process corroded 

 and stained the bone to such an extent that the details were 

 obscured. Centrosomes can be detected with considerable 

 clearness in gentian-violet preparations, but they are not distinct 

 in all cases. 



Considerable difficulty was at first encountered in obtaining 

 permanent mounts. Gentian violet has a tendency to fade if 

 mounted in balsam. Quite satisfactory results are obtained, 

 however, by mounting overstained preparations in balsam and 

 exposing them to light for a month or more. In ordinary prep- 

 arations the nuclear stain was well preserved, but the cyto- 

 plasmic coloration disappeared perceptibly. After some experi- 

 mentation it was found that almost perfect preservation can be 

 obtained in Canada balsam by observing the following pre- 

 cautions: 1) After dehydrating in absolute alcohol, wash in 

 several changes of benzol to remove all traces of alcohol. 2) 

 Mount in warm, thick, neutral balsam, I have several slides, 

 prepared according to this method over a year ago, in which the 

 staining is still almost perfect. It may be of interest to note that 

 preparations can be preserved in benzol indefinitely, even in 

 strong light, without fading. 



DESCRIPTION 

 Ratio of dividing to non-dividing cells 



In order to obtain a somewhat accurate idea of the ratio of 

 dividing to non-dividing cells, it is necessary to consider both 

 young and old bone. This study, however, yields only general- 

 ized results, since the ratio shows considerable range of variation 

 in different bones and in different regions of the same bone. 

 Thus in the ethmoid bone of an adult dog the greater number of 

 cells show no signs of division; in some fields none, while in 

 others as many as one cell out of every three are in some stage 

 of division. In the parietal bone of a twenty-one-day rat a con- 

 siderable range of cell age is encountered, and it shows that divid- 



