INDIRECT DIVISION 19 



hardened by cold or reagents, it is thought that mitoses go on to comple- 

 tion. Forty-eight hours may elapse before they entirely disappear from 

 the human body. 



Pluri-polar mitosis. Under abnormal conditions, as in the cancer 

 cells shown in Fig. 15, spindles may develop simultaneously in connection 

 with three or four centrosomes. Similar pluri-polar spindles have been 

 produced experimentally, by treating cells with various poisonous solu- 

 tions. An unequal distribution of chromatin may occur under such 

 conditions, and this may happen also with bipolar spindles, as shown in 

 Fig. 15, a. 



Number and individuality of the chromosomes. It is now generally 

 believed that every species of plant or animal has a fixed and characteris- 

 tic number of chromosomes, which regularly recurs in the division of all 



FIG. 15. MITOSES IN HUMAN CANCER CELLS. (From Wilson, after- Galeotti.) 

 a, Asymmetrical mitosis with unequal distribution of chromatin; b, tripolar mitosis; c, quadripolar" mitosis. 



its cells, with the exception of the germ cells, in which the number is 

 reduced. In certain species, however, the two sexes regularly differ from 

 one another in the number of their chromosomes, and one sex may con- 

 tain an odd number. Usually the number of chromosomes is believed to 

 be even. 



There is considerable difficulty in counting the chromosomes. Gener- 

 ally it is possible that some have been cut away in the process of section- 

 ing, so that, if the number is believed to be invariable, the highest number 

 found in any cell is assumed to occur regularly. Another source of error 

 lies in the fact that a bent chromosome may be counted as two, or rods 

 with their ends overlapping may appear as one. Farmer and Shove 

 have ventured to state that the number in Tradescantia "varies from 

 about twenty-six to thirty- three." Nemec found that twelve chromo- 

 somes occur regularly in young tissues of the onion, but that in older 

 tissues the number diminishes even to four. Sixteen have been recorded 

 in the onion by other botanists. Podophyllum is said to have sixteen 

 (Mottier), but Richards records counts of fourteen. In man the number 

 has been placed at 16 and 32, but it is now believed to be 24. Gutherz, 

 with particularly favorable material, emphasizes the difficulty of counting 



