INTERNAL CONDITIONS 493 



prepotency is associated with a tendency towards a preponderance 

 of one sex in the offspring. There do not seem to be any secure 

 data. 



So far as parental vigour may depend on what may be called 

 strained reproduction, Schultze's experiments on mice do not 

 in the least confirm the idea that this has any effect on the pro- 

 portions of the sexes. So far as parental vigour may depend on 

 deterioration supposed to result from close in-breeding, Schultze's 

 experiments are again adverse to the idea that this has any effect 

 on the proportions of the sexes. 



§ 5. Internal Conditions 



It remains to inquire whether, apart from parental conditions, 

 there are any definite peculiarities in the oogenesis and spermato- 

 genesis, in the processes of maturation, or in the fertihsation, 

 which can be associated with the production of male or female 

 offspring. Is any importance to be attached to the size of the 

 ovum, the age of the ovum, the freshness or staleness of the 

 spermatozoon, and so on ? 



Age of the Ovum. — It is held by some that if an &^^ is fertilised 

 soon after its liberation from the ovary, it is more likely to develop 

 into a female, and vice versa. Similarly, it has been held that 

 fresh spermatozoa tend to produce male offspring. But there 

 is no warrant for any such generalisation, though it is possible 

 that the relative age of the gametes may be one of the deter- 

 mining factors in certain cases. 



A side-light is thrown on this theory by Vernon's experiments 

 on hybridising sea-urchins, which seem to show that " the char- 

 acteristics of the hybrid offspring depend directly on the relative 

 degrees of maturity of the sexual products {Phil. Trans., Series B, 

 vol. cxc. (1898) pp. 465-529), and that the degree of staleness 

 of the ova and spermatozoa has an appreciable influence on the 

 development " {Proc. Roy. Soc. Ixv. [1899], pp. 350-60). 



