IS dependent on the age of the ovum at conception; 

 in other words, when conception occurs early in 

 heat most of the calves are heifers; late in heat 

 bulls. Other breeders are of the opinion that sex 

 IS dependent on the age of the spermatozoa, fresh 

 spermatozoa producing females. 



(b) The changes occurring during maturation of the 

 egg. (Seepage 133). 



(c) The influence of the nervous system. 



2. Those involving purely external factors: 



(a) The influence of nutrition, as in the case of plant- 

 lice, queen bees, and tadpoles. 



(b) The influence of temperature, moisture, seasons. 



3. Those involving the existence of two special groups of 



chromosomes following Mendel's Law of Heredity 



(page 81). 



This is the view most commonly held by geneticists. 



4. Those involving the influence of various factors, and 



it is probable that several of the factors mentioned 

 above may be concerned. It is possible, for ex- 

 ample, that " environmental changes may slightly 

 disturb the regular working out of the two possible 

 combinations that give male or female. Such dis- 

 turbances may effect the sex ratio but have 

 nothing to do with sex determination" (Castle). 



Some recent experiments suggest, however, that sex- 

 ratio can be influenced to some extent by selection. Miss 

 Helen D. King's investigations with albino rats brought 

 out the interesting fact that by selecting (1) a pair from 

 litters that showed the highest proportion of females, and 

 (2) a pair from litters showing the highest proportion of 

 males the sex ratio can be changed. Selection was caried 

 on in the two strains for about 25 generations. As a result 

 the proportion of females to males in the inbred female- 

 producing strain was 100 to 82,0. and that in the in-bred 

 male-producing strain was 100 to 117. (See Journ, Exp. 

 Zool. 1918, 27). 



While no breeding experiments have been carried on 

 with cattle with the direct object of producing female-pro- 

 ducing strains, many cases occur where bulls get more fe- 

 male than male calves. 



145 



