THE CARRIERS OF THE HERITAGE 17 



Throughout all the various cells that make up the 

 individuals of any one species these chromosomes 

 appear to be practically constant in number with some 

 exceptions to be mentioned later in connection with 

 sex. This law of the constant chromosome 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. 

 Species which apparently are closely related may 

 differ widely with respect to the number of their 

 chromosomes, while species of unquestionably re- 

 mote 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 

 peculiar 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 



