630 THE PHYSIOLOGY OF REPRODUCTION 



In the parthenogenetic Rotifer, Hydatina, Maupas ^ has 

 adduced strong evidence that if the parthenogenetic male eggs 

 are fertilised they are thereby converted into " winter " eggs 

 which give rise solely to females. If this is so (and Maupas's 

 conclusions are now generally accepted), it is a clear instance 

 of fertilisation altering the sex of the individual. It is stated, 

 however, that impregnation has no efiect unless it is performed 

 during the first few hours after hatching. 



Certain writers have adopted the view that sex in animals 

 generally is regulated by the time at which fertiHsation takes 

 place, that is to say, by the condition of the germ cells. Thus, 

 Thury ^ and subsequently Diising ^ expressed the opinion that 

 an egg which is fertihsed shortly after ovulation usually de- 

 velops into a female, and that an older egg tends to produce a 

 male. Thury claimed that he could regulate the sexes in cattle 

 by allowing coitus only at the beginning or at the end of the 

 cestrous periods, an early coitus being supposed to result in 

 the birth of a female, and a late coitus in the production of 

 a male, but other investigators have failed to estabhsh Thury 's 

 conclusions. 



Injluence of Nutrition. — Schenk * also has elaborated a highly 

 speculative theory which supposes sex to be determined by the 

 relative degree of " ripeness " or " unripeness " of the ovum ; 

 but the term " unripeness " is here used to imply a condition 

 consequent upon incompleteness of nutrition, while " ripeness " 

 is held to result from a more favourable state of nutrition. 

 " Ripe " ova are supposed to develop into males, and " unripe " 

 ones into females. The presence of sugar in the urine is evidence 

 of an incomplete metabohsm, and hence is regarded by Schenk 

 as implying a condition hkely to result in the birth of females. 

 By supplying women with a highly nitrogenous diet, which 



' Maupas, "Sur la Multiplication et la F^condation de I'Hydatina 

 senta," C. R. de I' Acad, dee Sci., vol. cxi., 1890. " Sur la F^oondation de 

 I'Hydatina senta," C. R. de I'Acad. des Sci., vol. cxi., 1890. "Sur la 

 D^terminisme de la Sexuality chez Hydatina senta," G. R. de I'Acad. des 

 Sci., vol. cxiii., 1891. 



^ Thury, Ueber das Gesetz der Erzeugung der Geschlechter, Leipzig, 1863. 



' Diising, " Die Regulierung des Geschlechtsverhaltnissesbei der Vermeh- 

 rung," &c., Jenaische Zeitschr., vol. xvii., 1884. 



* Schenk, The Determination of Sex, English Translation, London, 

 1898. 



