312 BIOLOGY AND ITS MAKERS 



tinct little rod-like bodies which make their appearance 

 during cell-division. These little bodies, inasmuch as they 

 stain very deeply with the dyes used in microscopic re- 

 search, are called chromosomes. And continued investigation 

 brought out the astounding fact that, although the number of 

 chromosomes vary in different animals (commonly from two 

 to twenty-four), they are of the same number in all the cells 

 of any particular animal or plant. These chromosomes are 

 regarded as the bearers of heredity, and their behavior during 

 fertilization and development has been followed with great 

 care. 



Brilliant studies of the formation of the egg have 

 shown that the egg nucleus, in the process of becoming 

 mature, surrenders one-half its number of chromosomes; it 

 approaches the surface of the egg and undergoes division, 

 squeezing out one-half of its substance in the form of a polar 

 globule; and this process is once repeated.* The formation 

 of polar globules is accompanied by a noteworthy process of 

 reduction in the number of chromosomes, so that when the 

 egg nucleus has reached its mature condition it contains only 

 one-half the number of chromosomes characteristic of the 

 species, and will not ordinarily undergo development without 

 fertilization. 



The precise steps in the formation of the sperm have also 

 been studied, and it has been determined that a parallel 

 series of changes occur. The sperm, when it is fully formed, 

 contains also one-half the number of chromosomes charac- 

 teristic of the species. Now, egg and sperm are the two ger- 

 minal elements which unite in development. Fertilization 

 takes place by the union of sperm and egg, and inasmuch 

 as the nuclei of each of these structures contain one-half of 

 the number of chromosomes characteristic of the species, 



* There are a few exceptions to this rule, as in the eggs of plant-lice, 

 etc., in \vhich a single polar globule is produced. 



