192 THE THEORY OF THE GENE 



separate, the members going to opposite poles; the 10 

 single chromosomes are distributed irregularly, without 

 division, to the daughter cells. Unfortunately the hybrid 

 is sterile, and cannot be used for further genetic work. 



The cross between two species of tobacco, Nicotiana 

 Tabacum and N. sylvestris, has been extensively studied 

 by Goodspeed and Clausen. Only recently, however, has 

 the chromosome number been determined; Tabacum has 

 24 (n=12) and sylvestris 48 (n=24) chromosomes. This 

 difference in chromosome number has not as yet been 

 correlated with the genetic results ; and the behavior of 

 the chromosomes in the maturation divisions has not 

 been reported. 



The hybrid from crossing these two species resembles 

 in every particular the Tabacum parent, even when that 

 parent is pure for factors that behave as recessives 

 toward the normal factors of the type Tabacum {i.e., in 

 crosses with varieties or races of Tabacum). Goodspeed 

 and Clausen interpret this result to mean that the Taba- 

 cum genes dominate as a group the sylvestris genes. They 

 have expressed this by saying that the ' ' reaction system ' ' 

 of Tabacum dominates the embryological processes of the 

 hybrid; or "the elements of the two systems must be 

 largely mutually incompatable. " 



The hybrids are highly sterile, but a few functional 

 ovules are formed. As the breeding results show, these 

 functional ovules are exclusively (or predominately) 

 either pure Tabacum or pure sylvestris. It may seem, 

 therefore, that in the hybrid only those (or largely only 

 those) ovules that contain a complete set (or nearly com- 

 plete set) of one or the other group of chromosomes are 

 functional. This view is based on the following experi- 

 ments. 



When the hybrid is fertilized with the pollen of sylves- 

 tris, a variety of forms is produced, among which there 



