the duration of the several mitotic stages in the 

 diyidinCt root-tip cells of the common onion. 



The ends sought in these studies are, (1) to devise and to prove an 

 accurate method for measuring the relative and aboslute average dura- 

 tions of the several mitotic stages in cell-division; (2) to make use of 

 this method in determining such durations for each of ten arbitrarily- 

 marked stages in the mitotic cycle of the dividing cells of the root-tips 

 of the onion {Allium cepa), at three different temperatures, namely, 

 10°, 20°, and 30° C, and thus to learn the effects of such temperature 

 increments upon the duration of the mitotic process as a whole and 

 upon each of its specifically marked stages, with the ultimate view 

 to aiding the analysis of the dynamics of mitosis. 



The text-books generally describe the mitotic sequence in consider- 

 able detail; but so severe and abnormal an environment for Uving 

 tissues are the microscopic shde and staining fluids that only recently 

 has special technique developed to the extent of permitting the direct 

 observation of mitotic changes. Especially difficult has been the 

 direct observation of mitosis in any cells other than the first divisions 

 in the transparent fertiUzed egg in a few organisms. Consequently, 

 most of the data descriptive of mitotic details have been secured from 

 dead samples. This has given a series of pictures of situations at the 

 several instants of killing, which when articulated have restored the 

 whole cycle in correct detail, with these special advantages, that up to 

 instants of kilhng the tissue may be living in practically normal envi- 

 ronment and the high staining may bring out mitotic details as yet 

 unseen in living cells. But this lack of data on the timing and meas- 

 uring of mitotic processes under definitely controlled environments 

 has prevented the building up of an extensive body of facts on the 

 dynamics of mitosis. The existing knowledge of mitosis is largely 

 descriptive of structure and structural changes. 



Ultimately, a process of better staining and viewing live cells may be 

 developed. It may then be possible to trace the normal and unham- 

 pered mitotic process in a single living cell and, from direct observa- 

 tion, to time the actual normal duration of each of its successive mitotic 

 stages, and thus from a large series of similar cells easily arrive at the 

 correct average relative and absolute durations of each stage; and 

 further, for the purpose of analysis, to time durations under definitely 

 governed and measured abnormal conditions. But for measuring the 

 velocities of normal activities, it is necessary, in the present stage of 

 development of microscopic viewing of living cells, to find some other 

 method of attack, one in which data are based upon mitotic processes 

 as nearly as possible normal and unhampered up to the instant of 



5 



