272 Cell as Basis oj Organic Activity 



two daughter cells gradually separate from one another. Each daugh- 

 ter cell now has exactly equal and identical kinds of chromatin material. 



The Duration and Cause of Mitosis. — The duration of the mi- 

 totic cycle varies with the condition of the organism, the type of cell, 

 and the temperature of the environment. The entire process may vary 

 in length from nine minutes in a Drosophila embryo to over 200 in some 

 other cells. 



As to the factors which initiate cell division, much remains to 

 be learned. It has been suggested that there is a relationship between 

 the surface area of the cell and the volume. As surface area does not 

 increase so rapidly as does volume, this may be important in the 

 metabolism of the cell. Possibly when the cell gets too large, it must 

 divide. Changes in electrical potential, viscosity of the protoplasm, and 

 surface tension may also be initiating devices. 



Amitosis. — This is another method of cell division in which 

 the nuclear configuration characteristic of mitosis is absent; that is, 

 spindle fibers and chromosome formation are not seen. This is actually 

 found only in abnormal cells, some transitory cells (such as those 

 found in embryonic membranes), and possibly a few protozoans. Can- 

 cer cells are characterized by the abnormal appearance of the chromatin 

 material. 



CHROMOSOME NUMBER 



The number of chromosomes, their shapes, sizes, and behavior 

 are all characteristic for a given species. They vary considerably from 

 one species to another, but are constant within any given species. 

 Chromosomes occur always in somatic tissue as homologous pairs ; one 

 chromosome of each pair originally was derived from the male parent, 

 the other from the female. 



The normal, double number of chromosomes is known as the dip- 

 loid number and is indicated briefly as 2N, with N equalling the number 

 of homologous pairs. This number is constant for any one species of 

 plant or animal. A few characteristic numbers are as follows : 



Crayfish 



Man 



Dog 



Horse 



Fruit fly (Drosophila) 



Roundworm (Ascaris) 



