Mar. II. 1918 Sterility in the Strawberry 661 



sylvesiris X N. tahacum the F^ generation resembled the A'', tabacum 

 parent and was partially fertile. From these he was able to obtain F3 

 plants apparently identical with the two parents and fully fertile. The 

 reappearance of types similar to the parents in large numbers in the 

 F3 generation suggests the elimination of gametes containing combinations 

 which would result in intermediate types. 



Detlefsen {10), working with animals, reports results obtained from 

 cavy crosses which may readily be explained on the basis of the elimina- 

 tion of certain combinations in the gametes of the males. He crossed 

 .tame females to wild males. The F^ males were all sterile. The cross F^ 

 female with wild male was not very successful and produced one sterile 

 male and a sterile female. The F^ females crossed to tame males gave 

 sterile males of which a few produced some nonfunctional sperms. The 

 females of this back-cross again crossed to tame males produced males 

 showing a low degree of fertility. As this process was continued, always 

 crossing back to tame males, the fertility of the male progeny increased 

 as they became more nearly homozygous for the tame condition. In the 

 sixth generation all of the males produced sperms, and 66.7 per cent of 

 these males were readily fertile. Apparently the more chromosomes of 

 one parent type which were present in the sperm, the greater its chance 

 of complete development. 



East {13) in a short abstract gives the conditions which he found in 

 the progency of the partially sterile hybrid Nicotiana rustica humilis X iV. 

 paniculata. The Fj progeny of this hybrid were very uniform, but only 

 I to 6 per cent of the female gametes were functional, and 2 to 6 per 

 cent of the pollen grains were morphologically perfect. In the V^ gener- 

 ation some perfectly fertile plants were found, many possible Fg combina- 

 tions were omitted, many more homozygous combinations occurring 

 than should be expected, and the parent types appeared once in everv 

 100 to 200 plants, whereas if all of the possible Fj combinations appeared, 

 the parent types would be much more rare. East considered that the 

 results might be explained on the basis either of selective elimination of 

 F2 zygotes or selective elimination of F^ nonfunctional gametes. He con- 

 sidered further that the elimination of the nonfunctional gametes might 

 be due to irregularities of chromosome distribution, which scheme seemed 

 improbable; or the facts might be interpreted without this assumption if 

 certain conditions were met which are as follows : 



If (i) there is a group of chromosomes in each parent that can not be replaced by- 

 chromosomes from the other parent; if (2) there is a group of chromosomes from each 

 parent, a percentage of which may be replaced by chromosomes from the other parent, 

 but where functional perfection of the gametes varies as their constitution approaches 

 that of the parental forms; if (3) there are other chromosomes that have no effect on 

 fertility and therefore can promote recombinations of characters in the progeny of 

 fertile Fg plants; if (4) a naked male nucleus entering the normal cytoplasm of the 

 egg in the immediate cross can cause changes in the cytoplasm that will affect future 

 reduction divisions; if (5) this abnormally formed cytoplasm is not equitably dis- 

 38325°— 18 4 



