INTERNAL CAUSES 527 



inheritable are caused internally. The cause may be gome 

 peculiar feature occurring in connection with the formation of 

 sex cells. It may be due to combinations of factors in fertiliza- 

 tions, or to alterations in cell substances due to unknown causes. 

 The formation of sex cells is preceded by the reduction division 

 as a result of which the sex cells (eggs and sperms) have only half 

 as many chromosomes as the other cells of the parent. When an 

 egg and a sperm fuse in fertilization, the normal number is again 

 established, and the individual developing therefrom really has 

 two sets of chromosomes, one set having been introduced by the 

 egg and therefore having come from the mother parent, and the 

 other set having been introduced by the sperm and therefore 

 having come from the father parent. Each chromosome consists 

 of many granules or particles each of which ip thought to be re- 

 sponsible for the development of a character. The set of mother 

 chromosomes carries the particles or factors responsible for the 

 development of the mother characters, while the set of chromo- 

 somes from the father parent carries the factors for father char- 

 acters. In the reduction division, which occurs when the indi- 

 vidual forms sperms and eggs, father and mother chromosomes 

 carrying factors for the same kind of character are supposed to 

 pair and then separate, the father chromosome of the pair going 

 to one of the new cells and the mother chromosome of the pair 

 going to the other of the new cells. Of course each new cell 

 usually gets some father and some mother chromosomes. The 

 members of each pair of chromosomes go to different cells, no 

 matter as to which cell they go. Since chromosomes containing 

 factors for the same character go to opposite cells, the mother 

 chromosomes containing factors for color of flower, length of stem, 

 and so on go to one cell and the father chromosomes having factors 

 for these same characters go to the other cell. This means that 

 the sex cells have only the father or only the mother factors for a 

 certain charatiter. They do not contain both, but either one or 

 the other. The sex cells are said to be pure in respect to any of 



the characters. 



Since the sperms and eggs usually get some father and some 

 mother chromosomes, it is obvious that some of the father factors 

 for certain characters and some of the mother factors for certain 

 other characters become associated in the sperms and eggs. The 

 father and mother factors may be associated in all possible ways. 



