Heredity — Genetics 675 



The evidence that inheritance is due to chromatin may be shown by 

 the following facts: (1) Of all the parts of a cell, the chromatin is the 

 most constant portion through all the changes of the cell. This sug- 

 gests the chromatin as the probable carrier of hereditary factors. (2) 

 The chromosome complex is maintained throughout the process of cell 

 division (mitosis). At this time the mechanism for the transmission of 

 hereditary factors from one cell to its daughter cells must be present 



Fig. 332. — Chromosomes of the fruit fly (Drosophila sp.), showing pairings, 

 sizes, and shapes in the male and female (diagrammatic). In the female thenr 

 are three pairs of autosomes and a pair of straight X chromosomes. In the male 

 there are three pairs of autosomes and one straight X chromosome and a hook 

 shaped Y chromosome. 



B 



>tO>IO»ll 



(CfCKOUiMjc* 



Fig. 333. — The chromosomes of man. A, Spermatogonium (of the male) dur- 

 ing division, drawn so that the chromosomes do not overlap; B, the twenty-four 

 pairs of chromosomes from a somatic (body) cell. (From Lindsey: A Textbook 

 of Genetics. By permission of The Macmillan Company, publishers.) 



and functional; otherwise there could be no such transfer. (3) There 

 is a close correlation between abnormal inheritance and abnormal 

 chromatin behavior. (4) There is a close relationship between chromo- 

 somes and the determination and development of sex, the latter being 

 a hereditary character. The fact that the number of chromosomes is 



