218 GENETICS 



pear to be practically constant in number with some 

 exceptions to be mentioned later in connection with sex. 

 This law of the constant chomosome number for any 

 species was first stated by Boveri in 1900. 



The chromosomes of different organisms vary in 

 number from two in the worm Ascaris up to perhaps 

 1600, according to Haecker ('09), in certain radiolaria. 

 A recent list records the number of chromosomes typical 

 for 960 different animals. 1 Species which apparently 

 are closely related may differ widely with respect to the 

 number of their chromosomes, while species of unques- 

 tionably remote relationship may have an identical 

 number of chromosomes in each of their cells. The 

 number of chromosomes characteristic for a species, 

 therefore, is in no way an index to the complexity or 

 degree of differentiation of the species. 



Besides the nucleus there may often be identified in 

 the cytoplasm of the animal cell a tiny body known as 

 the centrosome. At certain times in the life-cycle of 

 a cell the centrosome becomes the focal point of pecul- 

 iar radiating lines, which play an important part in 

 the behavior of the cell, particularly during the period 

 of division. 



Every cell passes through a cycle of life which may 

 be compared with that common to individuals. It is 

 born from another cell; passes through a vigorous 

 youth characterized by growth and transformation; 

 attains maturity when the metamorphoses of its earlier 

 life give place to a considerable degree of stability ; and 

 finally, after a more or less extended period of normal 

 Vowr. of Morphology, vol. 34, pp. 1-67, 1920. 



