464 THE MAINTENANCE OF SPECIES 



tissues of the body. In other words, while there is accompUshed an 

 equal distribution of chromosomes, an unequal distribution and 

 elaboration of the cytoplasm takes place. It is plain, therefore, that 

 in this process the genes not only take in material to be elaborated 

 from the cytoplasm, but that in turn something must go out from 

 them to bring about the differentiation of the surrounding cytoplasm. 

 That there is a chemical difference between what is in the chromo- 

 somes and what is outside of them is proven by the differential way 

 in which these substances respond to certain stains. Apparently 

 there is carried on from generation to generation throughout life an 

 elaborate and extensive performance of "give and take" between the 

 germplasmal chromosomes and the somatoplasmal cytoplasm of the 

 cells. Dr. H. S. Jennings states the matter in the following words : 



" This process of changing the cytoplasm by the action of the genes is the 

 fundamental thing in development. The genes repeat this process over and 

 over again, taking in cytoplasm, modifying it, giving it off in changed condi- 

 tion, and leaving the genes themselves unaltered." ^ 



In the light of the intimate relationship between genes and cytoplasm, 

 and recognizing the dominant part taken by it in developmental 

 processes, can we assign any truly hereditary role to the cytoplasm 

 itself, except as it is first taken in and made a part of the chromosome 

 complex ? 



It is common knowledge that apple blossoms, when fertilized with 

 foreign pollen, produce only apples like the maternal parent because 

 the apple itself is merely an elaboration of the maternal tissue of the 

 ovary, determined in its character before the ovule in the ovary is 

 fertilized. Seeds of such apples, however, grow into trees that 

 produce fruit showing paternal as well as maternal characters. This 

 sort of "maternal inheritance" suggests the presence of some heredi- 

 tary factor outside the genes that keeps an apple a sweet apple, for 

 instance, although its blossom is fertilized by pollen from a sour 

 apple tree. It is only necessary, however, to remember that the 

 cytoplasm of the sweet apple is already determined by the germinal 

 contributions of the preceding generation, both maternal and paternal, 

 rather than by the fertilizing pollen in the present case, in order to 

 find a satisfactory explanation that does not involve cytoplasmic 

 determination. 



' From Jennings, H. S., Genetics, p. 233. By permission of W. W. Norton & Company, publishers. 



