EFFECTS OF INBREEDING ON THE SEX RATIO 35 



spermatozoa from the first animal are functional when used with 

 ova of another animal. Morgan ('14) has suggested that the 

 infertility of the eggs of Ciona to spermatozoa from the same in- 

 dividual may be due to the similarity in the hereditary complex 

 of the germ cells which in some way decreases the chances of 

 their uniting. The selective fertilization experiments made by 

 Marshall ('10) with different varieties of dogs and also my own 

 experiments with different varieties of rats show^ that the ova of 

 these animals have a strong tendency to unite with spermatozoa 

 from indi\'iduals belonging to unrelated stock rather than with 

 spennatozoa from individuals of the same 'blood. ' When my own 

 experiments are completed the results will show, I hope, whether 

 there is a still more delicate chematactic reaction between the 

 ova and the spermatozoa w^hich will lead to the production of 

 more males than females among the hybrid offspring. The ano- 

 malous sex ratios that appear in F, hybrids almost invariably 

 show an excess of males. This suggests that the greater the dif- 

 ference between individuals as regards theis blood relationship the 

 stronger is the attraction between the ova and the 'male-pro- 

 ducing spermatozoa. If this suggestion proves true, its converse 

 ought also be to true, and in a closely inbred line we w^ould expect 

 that the chemotactic reaction betw^een the ova and the sperma- 

 tozoa would be such that an excess of females would be produced. 

 Such a possibility is not incompatible with the results of the pres- 

 ent investigation, since in the inbred strain, as a whole, the sex 

 ratio was below the norm, while the sex ratios in the litters of the 

 female line (B) showed a greater deviation from the norm than 

 did the sex ratios in the litters of the male line (A) . 



The results of this series of experiments, as a whole, seem to 

 indicate that in the rat, as in the pigeon (Riddle, '14, '16, '17), 

 in Drosophila (Moenkhaus, '11) and in the guinea-pig (Papani- 

 colau, '15), the female has more influence in determining the sex 

 ratio than has the male. Yet it is not in the differentiation of the 

 ova, nor in the development of the spermatozoa, that the key to 

 the riddle of sex-determination will be found. A knowledge of 

 the interaction of the germ cells, and of the conditions that in- 

 fluence it, must be gained before the final solution of this problem 

 can be attained. 



