34 



Heredity and Euti^enics 



viduals, one characterized by tlie reduced number of chro- 

 mosomes and bearing the sex organs (hence called the 

 gametophytc), the other characterized by the doubled num- 

 ber of chromosomes and producing spores (hence called 

 the sporopliytc). In general, it may be said that among 

 animals reduction occurs in connection with gamete forma- 

 tion, while among plants it occurs 

 in connection with spore formation. 

 To use the simpler illustration 

 of animals, it is evident that the 

 cells making up the body of an 

 individual contain the doubled 

 number of chromosomes (Fig. 6, 

 i), derived from the fertilized egg 

 from which the body developed. 

 Wlien the sexual cells (eggs or 

 sperms) of this individual are 

 formed, the reduction occurs, and 

 the eggs or sperms contain the re- 

 duced number. How the maternal 

 and paternal chromosomes are dis- 

 tributed in this reduction may not 

 be clear, but it is evident that 

 either eggs or sperms may contain chromosomes derived 

 from either line of descent (Fig. 6). In other words, it is 

 not inconceivable to think of an egg containing only chro- 

 mosomes derived from the paternal side, or of a sperm con- 

 taining only chromosomes derived from the maternal side, 

 or most likely of both eggs and sperms containing chromo- 

 somes derived from both sides (Fig. 6, 2-j) . 



These considerations indicate how every sexual fusion 

 results in a complex of possibilities, and the study of heredity 



Fig. 6. — Diagram illustrat- 

 ing result of reduction: i, 

 ordinary cell of the body con- 

 taining doubled number of 

 chromosomes; 2-5, eggs or 

 sperms containing the reduced 

 number, and illustrating the 

 possibilities in the distribu- 

 tion of paternal (A A) and 

 maternal (BB) chromosomes. 



