CHROMOSOMES THE BEARERS OF HEREDITY 627 



genes or determiner sof the qualities which maybe passed from parent 

 to offspring ; in other words, the qualities that are inheritable. 



The germ cells. It has been found that certain cells of the body, 

 the egg and the sperm cells, before uniting usually contain only 

 half as many chromosomes as do the body cells. 1 In preparing 

 for the process of fertilization, half of these elements have been 

 eliminated by reduction division (diagram, p. 630), so that when the 

 egg cell and the sperm cell are united they will have the same 

 number of chromosomes as the other cells of the body. 



We have already learned that in the process of fertilization the 

 nuclei of the sperm and of the egg cell unite, or fuse, to form a new 

 nucleus in the fertilized egg. This fertilized egg will contain an 

 equal number of chromosomes from each parent. Since the 

 inheritable characters are contained in the chromosomes, both 

 parents will hand down an equal number of characters to their 

 offspring. In this way characters from each parent are handed 

 down to the new individual. 



Investigations of heredity have centered, in recent years, on 

 the composition and action of the chromosomes. It has been 

 found that they differ in number according to the species of the 

 animal. In a little worm called Ascaris and in the fruit fly there 

 are only four chromosomes in each body cell; in the mosquito 

 Culex there are six, in the rat sixteen, in the frog twenty-six, in 

 certain crustaceans more than one hundred and fifty, in one spider 

 a hundred and sixty-eight. In some animals, as has been shown, 

 the number differs with sex. Man is believed to have forty-eight. 



Professor Morgan, of the Institute of Technology, Pasadena, 

 California, has found, as a result of investigations on the fruit fly 

 (drosophila) , that each chromosome is actually composed of genes 

 or the inheritable material that represents unit characters, and 



1 This is not quite exact, for it has been found that in some animals at the time 

 when the chromosomes are reduced in number, there is an even number in the 

 female sex cells but an odd number in the male sex cells. When the male cells 

 divide to reduce the number of chromosomes, some sperm cells receive an odd 

 and some an even number of chromosomes. Therefore, after fertilization, some 

 eggs have an even and some an odd number of chromosomes. The fertilized egg 

 cells with the odd number of chromosomes develop into male animals ; the cells 

 with the even number of chromosomes become females. The sex-determining 

 chromosome is known as the accessory chromosome and is found in some worms, 

 many insects, myriapods, spiders, and some mammals, including man. 



