CHAPTEB XI 



THE REDUCTION OF THE CHROMOSOMES 



The subject of chromosome reduction is one of the mosl important to 

 be met with in the study of cytology. Many of the problems, both theo- 

 retical and practical, upon which biological investigators are expending 

 their most intense efforts seem to be bound up directly or indirectly with 

 the reduction of the chromosomes. The essential feature of reductioD is 

 relatively simple in nature, and must be thoroughly grasped in order that 

 the discussions in the following chapters may be intelligible. The entire 

 process by which reduction is accomplished, on the other hand, is very 

 complicated and extremely difficult to observe and interpret with any 

 degree of confidence. In spite of the enormous amount of work already 

 done there still exists much difference of opinion regarding some of the 

 significant steps in the series of changes undergone by the nuclear material. 

 In the present chapter a number of these opinions will be reviewed, but 

 our main purpose will be to make clear the fundamental feature of chro- 

 mosome reduction. 



We have seen that all the cells of the body in a given species are char- 

 acterized by the presence of a certain number of chromosomes in their 

 nuclei, and that this number is held constant throughout development by 

 an equational division of every chromosome at every somatic mitosis. 

 When we speak of "reduction" we ordinarily refer to the fad that at a 

 certain stage in the life history of the organism the number of chromosomes 

 is reduced one-half. This mere change in the number of chromosom 

 though very important, is not in itself the essential feature of the reducing 

 process, as will be seen further on. The whole number is restored at the 

 time of fertilization, when two nuclei, each with the reduced number, 

 unite. In all organisms reproducing sexually reduction and fertilization 

 thus represent the two most critical stages in the life cycle so far as the 

 chromosomes are concerned; hence the exhaustive researches on these two 

 processes. 



Discovery. — The discovery of reduction was made by van Beneden, 

 who in 1883 announced that the nuclei of the egg ;md spermatozoon of 

 A scaris each contain one-half the number <>!' chromosomes found in the 

 body cells. Although van Beneden and other early workers believed that 

 the change in number was brought about by the simple casting out of half 

 the chromosomes during the growth of the germ cells, it was soon shown 

 that this view was incorrect, and that " reduction w effecU <l by n rearrangi - 



21 '.» 



