58 THE CELL 



of two daughter cells. The nuclear membrane reappears, as does also 

 the nucleolus. Each daughter cell is thus provided with a resting 

 nucleus. The fact that the number of chromosomes which enter 

 into the formation of the chromatic reticulum of the nucleus of 

 each daughter cell is the same as the number into which the 

 spireme of the parent cell divided, has suggested the hypothesis 

 that the chromosomes maintain their identity even during the 

 resting stage. 



The time required for the mitotic process is usually from one-half 

 to three-quarters of an hour. Exceptionally it is prolonged to several 

 hours. 



The role which each part of the cell plays in the vital activities of the cell,, 

 and the physiological correlation of these various parts, have been but partially 

 determined. Experiments upon some of the protozoa show that if the cell be 

 divided into two parts, one part containing the nucleus, the other part non- 

 nucleated, the behavior of the two parts is very different. The part contain- 

 ing the nucleus soon again becomes a complete cell with all the properties which 

 the cell originally possessed. The non-nucleated part responds somewhat to 

 stimulation, is capable of some movement, makes some feeble attempts at 

 digestion, but is incapable of secretion, of reconstructing the complete cell, of 

 reproduction, and soon dies. Even when a small part of the nucleus remains 

 in the cut-off piece of cytoplasm reconstruction may take place. On the other 

 hand a nucleus completely deprived of its cytoplasm is incapable of reconstructing 

 the cell. In Infusoria each cell has two nuclei, a macro-nucleus and a micro- 

 nucleus, the former connected with nutrition, the latter with reproduction. 

 These facts together with the behavior of the nucleus during the ingestion of 

 food, during secretion, and even during motion, warrant the belief that while 

 in all probability most of the actual work of the cell takes place in the 

 cytoplasm, the nucleus exerts a more or less controlling influence over all cell 

 activities. 



The parts which the several cell structures play in mitosis have been the 

 subjects of much study and are as yet not fully determined. 



As to the behavior of the chromatic portion of the mitotic figure little doubt 

 exists. It originates in the chromatic portion of the nuclear reticulum of the 

 parent cell and its destination is the chromatic portion of the reticulum of the 

 daughter cells. 



The role of the centrosome in mitosis is not so clear. It has been called the 

 "dynamic centre" of the cell because in most cases it appears to be the active 

 agent in initiating and probably further directing the mitotic process. The 

 origin of the astral fibres is not always the same. In Infusoria the centrosome 

 is found within the nucleus and both amphiaster and chromosomes are of nu- 

 clear origin. In some of the higher plants the amphiaster is derived wholly 

 from the spongioplasm. In the egg cells of Echinoderm, part of the amphiaster 

 (central spindle) is of nuclear, the remainder (asters) of cytoplasmic origin. 

 That the centrosome is not always the active factor in mitosis is shown by the 



