CHAPTER XVI 



THE CHROMOSOME THEORY OF HEREDITY 



The rcmarkal)le behaviour of the chromatin during mitosis, 

 described in Chapter \T, is fundamental evidence that this sub- 

 stance plays a peculiar and important part in the history of cells. 

 During mitosis all other portions of the nucleus become merged 

 with the cytoplasm, the centrosome plays a part which seems to 

 be limited entirely to cell reproduction, and the cytoplasm and 

 chromatin are equally divided between the daughter cells. The 

 division of the cytoplasm is attended by no definite phenomena 



b ^ d T f ^ T : 



f 



Fig. 166. — Examples of one of the chromosomes of Phrynotetlix sp. taken 

 from thirteen individuals. The dotted line passes through homologous 

 granules, x 2200. (From IMcClung, in Cowdry's General Cytology, after 

 Wenrich; with the permission of the University of Chicago Press.) 



which would indicate both quantitative and qualitative equality 

 of distribution. The chromosomes, however, are developed in 

 many cases as slender structures showing longitudinal differentia- 

 tion which is evident in well prepared specimens (Fig. 166) and 

 the longitudinal splitting that occurs in the metaphase is appar- 

 ently an equation division. Moreover in the many successive 

 mitoses intervening between the union of the germ cells and the 

 completion of embrj'ological development extensive cytoplasmic 

 differentiation occurs. Although it is accompanied by some 

 nuclear differentiation evidence shows that the same chromosome 

 complex persists even in highly specialized cells. This complex 

 is characteristic of the different parts of the body of all individuals 

 of the species, and in some cases of related species. McClung's 



289 



