Internal Factors of Sex-Determination 393 
toth, rath, 14th, 15th, 16th, 21st, 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, Diising, 1838) that if the egg is fertilized soon after leaving 
the ovary it zends 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 11 months 12 litters, containing 42 
males and 34 females. A third produced in 3} months 6 litters, 
containing 20 males and 11 females. A fourth produced in 43 
months § 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. 
