42 GENERAL ZOOLOGY 



as has been described. The purpose of mitosis is appar- 

 ently to secure the accurate division of the chromosomes 

 so that each daughter cell may receive exactly the same 

 amount of chromatin. The chromosomes are believed to 

 be the chief bearers of hereditary qualities and have been 

 known as the " vehicles of inheritance." This perhaps 

 explains why such care is taken to insure accurate division. 

 There is a characteristic number of chromosomes for 

 each plant or animal and when one of its cells divides the 

 same number is always reformed from the scattered chro- 

 matin of the nucleus. Thus the number in several animals 

 is as follows: 



Ascaris, a nematode 2 or 4 Rana, a frog; and man 24 



Aphis, a plant louse 8 Lumbricus, an earthworm 3% 



Gryllotalpa, a cricket 12 Crepidula, a snail 60 



Rat 16 Artemia, a brine-shrimp 168 



It is known that species in the same genus show constant 

 differences in the number of chromosomes. Each shows its 

 characteristic number generation after generation. Some- 

 times there is an odd chromosome. This peculiarity is 

 particularly characteristic of the sex cells of certain animals. 

 In such cases, the minute male sex cell, or spermatozoon, 

 may carry the odd chromosome and is believed to produce 

 a female when it "fertilizes" an egg; one which has none 

 will give rise to a male. The odd chromosome, therefore, 

 perhaps serves to determine sex. 



AMITOSIS OR INDIRECT CELL-DIVISION 



When the cell divides without mitosis the process is very 

 simple. The nucleus elongates, becomes constricted in the 

 middle like a dumbbell, and finally is pinched in two (Fig. 

 26). The two daughter nuclei may differ greatly in size 

 and. in the amount of chromatin they contain. Amitosis 

 often takes place in old, weak, or degenerate cells and may 

 be looked upon, at least in some cases, as a sign of a general 

 wearing out of the cell mechanism. Some students of cell 

 physiology dispute this interpretation, however. 



