Internal Factors of Sex-Determination 393 



loth, 1 2th, i/j.th, 1 5th, i6th, 2ist, and 28th weeks. In none of 

 these cases was there found any evidence in favor of the view that 

 the sex of the first-born young is affected by the age of the parent. 



Condition of the Germ-cells 



It has been claimed in the case of certain mammals (Thury, 

 1863, Busing, 1838) that if the egg is fertilized soon after leaving 

 the ovary it tends to produce a female ; if not fertilized till later, 

 it tends to produce a male. Thury based his conclusions on 29 

 experiments with cows, in which union took place at the begin- 

 ning or at the end of the period of heat. Others have failed 

 to confirm his conclusions, and contradictory results have been 

 obtained with rabbits and hens. The same argument has been 

 applied to the condition of the sperm, but there is no clear evi- 

 dence in its favor. It has been pointed out that such a view is 

 contradicted in the case of bats, in which the spermatozoa, re- 

 ceived by the females, may remain alive for months before the 

 eggs are set free; and yet the same proportion of males and 

 females found in other animals are produced. In the case of 

 the bee the sperm is stored up. in the seminal receptacle of the 

 queen for several years ; yet in all cases the fertilized eggs pro- 

 duce only females. 



Vigor of the Parents 



Schultze has carried out an experiment that goes to show that 

 continued or strained reproduction on the part of the female 

 does not affect the proportion of males and females. As soon 

 as the young mice were born they were removed and the mother 

 again produced a new litter. One female produced in a year 

 and 52 days 14 litters, containing 52 males and 53 females. An- 

 other female produced in n months 12 litters, containing 42 

 males and 34 females. A third produced in 3^ months 6 litters, 

 containing 20 males and n females. A fourth produced in 4 

 months 5 litters, containing 19 males and 22 females. The total 

 is 133 males and 120 females, which for the numbers involved 

 is a fair approximation to equality. 



