I02 ZOOLOGY 



of the germ cells which are housed in that body. Nor do they 

 mean that the environment may not operate to change germ 

 plasm. They only mean that we have no sufficient evidence 

 for believing that we can change germ plasm in a specific way 

 by changing the body of the parent in that particular way. 

 For example, training parents in speaking English or in playing 

 the piano apparently does not in any degree make either of 

 these performances easier for their children. It is unquestioned, 

 however, that qualities received Ijy inheritance from the pre- 

 ceding generations are, under favoring circumstances, capable 

 of being transmitted to the following generations. Whether 

 they are actually so transmitted depends upon the circum- 

 stances. Chapter XXVI discusses some of these circumstances. 



131. The Bearers of Heredity. — It follows from the fact 

 that the adult organism is produced from one or more reproduc- 

 tive cells that these cells are in some way endowed to carry the 

 hereditary qualities. These cells are all that pass from the old 

 generation to the new. There are strong evidences that the 

 chromatin elements, or chromosomes, in the nuclei of the sperm 

 cells and ova are the most important material structures by 

 means of which transmission is effected. The chromosomes of 

 the fertilized ovum are contributed equally by the male and 

 female cells, and they are the only structures in the sperm and 

 ovum which are apparently equal in amount. This, taken in 

 connection with the fact that one parent does not seem, on the 

 average, to have any more power than the other to influence 

 offspring, furnishes a basis for the belief that the chromosomes 

 are the physical basis of heredity. 



Some students of the subject believe that the cytoplasm is, 

 equally with the chromosomes, a carrier of hereditary qualities. 

 They think that cytoplasm is responsible for those qualities 

 that both sexes have in common. That is, the cytoplasm con- 

 serves the common characters of the species. The chromo- 

 somes are held to be more responsible for those qualities in 

 which the individuals of a species differ, as sex, size, color, 

 vitality, etc. Recent investigations show that the paternal and 

 maternal chromosomes tend to retain their distinctness and to 

 be equally distributed to all the nuclei of the developing embryo. 



