CELLS IN DIVISION 



sufficiently clear-cut to serve as a means of measuring the duration of 

 the mitotic cycle. More usually, mitotic rhythms are less marked and 

 take the form of an increased proportion of cells in division at particular 

 periods of the day. These periodicities have been described both in 

 animals and in plants (Tischler^^). Friesner^°° studied the growth of 

 the roots of several flowering plants in darkness and at constant tem- 

 peratures, and thus eliminated the normal diurnal changes in environ- 

 mental conditions. Rhythmic variations in the number of mitoses in 

 the root-tip were still detectable, and a period of enhanced cell division 

 alternated with one in which enlargement of cells was more prominent. 

 The times at which these maxima and minima were found to occur 

 were not related to the hour of the day, but only to the time at which 

 germination had been begun, and were thus governed by factors 

 intrinsic to the growth of the plant. No evidence of mitotic rhythms in 

 root meristems were found by Winter^^^ in Gladiolus, by Gray and 

 ScHOLES^^ in Vicia, or by Brown^^^ in Pisum. The latter author states 

 that the 'exclusion of light [is] an important condition for the elimina- 

 tion of mitotic periodicity.' 



The mitotic rhythms in animal tissues seem to be related to environ- 

 mental conditions. Bullough has clearly demonstrated that in the ear 

 epithelium of male laboratory mice, cell division is at a maximum when 

 the animals are at rest, and at a minimum when they are awake and 

 active ; the mitotic peaks observed by Bullough are thus related to the 

 daily routine in the animal house of his laboratory.* Further researches 

 by this author have suggested that glucose is transferred from the 

 blood to the tissues while the animal is at rest, and that the extent of 

 mitotic activity in the ear epithelium is related to the concentration on 

 the available blood sugar (p 187). It is possible that a diurnal mitotic 

 rhythm in the eight-day-old human epidermis may be governed by the 

 same factors, according to the results which Cooper and Schiff^^ 

 obtained by counting mitosis in the excised prepuce removed at 

 different periods during the twenty-four hours. Considerably more 

 mitoses were found in the foreskin of a child when it had been circum- 

 cised at night. 



STRUCTURE OF CHROMOSOMES 



At metaphase, the general shape of most large chromatids in plants or 

 animals is that of cylindrical threads divided into equal or unequal 

 limbs by a constriction at the site of the attachment to the spindle. 

 Several names for this have been used; Darlington's^^^ term 'centro- 

 mere' is now usual in this country, while 'kinetochore' (Sharp^°^) is 

 often used by American cytologists. Instances of terminal centromeres 



* In the bone marrow of the mouse, Milletti^"^* has recently found a mitotic peak at 

 4 a.m., and a minimum at mid-day. 



91 



