HEREDITY 



311 



200). Each cell now contains adjacent to the centrosome the 5 

 number of p and m chromosomes. The telophase is a period of 

 nuclear reconstruction. The nuclear membrane appears and within 

 it the chromosomes lose their distinctness, the chromatin becoming 

 again a spireme, then scattering and remaining so until the prophase 

 of the next mitosis. 



It has been found that in addition to the nuclear phenomena that 

 characterize mitotic cell divisions, there are distinct changes in the 

 physical nature of the cell contents. By centrifuging cells at various 

 stages of the division cycle and noting 

 the effect of centrifugal force in dislo- 

 cating granules in the cell it became 

 apparent that the viscosity of the cyto- 

 plasm changes during the cycle. At 

 the time of the onset of separation of 

 the two daughter cells the viscosity is 

 greater than at any other period of the 

 division cycle. By introducing exceed- 

 ingly fine glass needles into the cell it 

 was found that the whole mitotic 

 spindle is considerably more rigid 

 than its surroundings; it can be 

 pushed about with the needle. From these and other facts it is 

 perfectly clear that mitotic cell division involves profound changes 

 in the metabolism and the physical nature of the cell substance; the 

 process is not solely the changes which, with the aid of the micro- 

 scope, may be observed to occur in the nucleus. But our interest 

 here must be confined entirely to the nuclear phenomena. 



Mitosis is a most striking and regular performance and one who 

 observes it cannot but be impressed with the equal distribution of 

 the chromatin and the preservation of the identity of each p and m 

 chromosome as synaptic pairs in the daughter cells. And so the 

 quantity of chromatin and the identity of the chromosomes are 



Fig. 200. — The telophase. 



