Experimental Growth in Animals 217 



2. The intermitotic period is much longer than the duration of the 

 experiment, and is not modified by the cxj^eriment. 



3. The arrested mitoses are not destroyed before the moment the 

 tissues are fixed and examined. 



4. The tissue is homogeneous from the point of vie\v of mitosis, that 

 is to say, mitotic rates and intermitotic periods do not vary from 

 one region of the tissue to another. 



5. The mitotic rate does not \ary chning the experimental period, 

 in control animals. 



Such conditions are not often fulfilled. One type of experiment in 

 which they are is liver regeneration; this will be considered further. 

 In mammals, cellular destruction is a factor which cannot be ignored. 

 If, however, the above-mentioned causes of error do not exist, the 

 average duration of mitosis can be found by the formula A = Mt/X, 

 in which M is the mitotic index before colchicine, and X the index 

 found t hours after the injection of the alkaloid. 



If this formula is applied to the resvdts obtained in the experi- 

 ments referred to in the previous paragraph,- it is found that after 

 "theelin" stimulation, the average duration of mitoses would be 10 

 minutes. This is a remarkably short period, and it may be questioned 

 whether mitoses can be completed so rapidly. However, results ob- 

 tained by A. P. Dustin, Sr., in the uterus of the rabbit after stimula- 

 tion by chorionic gonadotropic hormones, are rather similar.^^ The 

 increase in the ninnber of mitoses was observed in repeated biopsies. 

 Figure 9.2 shows that it was considerable, and that in one animal, 

 the calculated duration of each mitosis, had it not been arrested by 

 colchicine, would be 12 minutes. These results bring some evidence 

 for mitotic stimulation, for the prophase mitotic index increased also. 

 This indicates that more cells were undergoing prophase than ex- 

 pected; that is to say, a true stimulation took place. This index rose 

 from 7.56 to 14.8 in 2 hoins, and from 4.8 to 24.4 in 7 hours. It must, 

 of course, be supposed here that the duration of each prophase was 

 not affected by colchicine. 



Such results are rather complex, for the mitotic index could have 

 been modified by the traumatisms of the biopsies themselves, and also 

 by the continued action of the hormone. The possibility of a synergic 

 action of hormones and colchicine cannot be rided out'^'^ (cf. Chapter 



The following results-^' are all the more interesting, for while 

 they apparently could demonstrate such a synergism, a much simpler 

 explanation is possible. Table 9.1 gives the results of mitotic counts 

 in the seminal vesicles, after stimulation by a single large dose of 

 testosterone. There appears to be a veritable "explosion" of mitoses, 

 to use the expression coined by A. P. Dustin, Sr. Does this give evi- 

 dence of mitotic stimulation by the alkaloid? The counts of the con- 



