270 HEREDITY AND ENVIRONMENT 



processes. What are these units in terms of 

 cell structures and where are they located in 

 the cell? 



We have in the chromosomes, as Wilson 

 especially has emphasized, an apparatus which 

 fulfils all the requirements of carriers of 

 Mendelian factors (Fig. 57). Both factors 

 and chromosomes come in equal numbers 

 from both parents ; both maternal and paternal 

 factors and chromosomes pair in the zygote 

 and separate in the gamete as shown in Fig. 

 56 and Fig. 57; and so far as is known the 

 chromosomes are the only portions of the germ 

 cells which fulfil these conditions. Further- 

 more there is much additional evidence that 

 the chromosomes are especially concerned in 

 heredity, as was pointed out in the last chapter, 

 and it is not reasonable to suppose that this 

 remarkable coincidence between the distribu- 

 tion of Mendelian factors and of chromosomes 

 is without significance. 



Of course Mendelian factors are not the only 

 factors of development but merely the differ- 

 ential factors which cause, for example, one 

 guinea-pig to be white and its brother to be 



