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 which a single polar globule is produced. 
