668 THE PHYSIOLOGY OF REPRODUCTION 



the individual. It is stated, however, that impregnation has no 

 effect unless it is performed during the first few hours after- 

 hatching. Moreover, parthenogenetic female eggs are also produced. 



In recent years a large number of further experiments have been 

 made with this rotifer by Whitney 1 and also by Shull, 2 in an 

 attempt to determine the conditions controlling sex-production. 

 Whitney has been able to change a pedigree stock of Hydatina senta 

 from a parthenogenetic female-producing variety, kept on Polytoma, 

 to a male-producing sexual stock by altering the diet to Ghl<n/u/- 

 domonas. This change, as Doncaster has pointed out, however, is 

 not a real sex change but rather a change from one mode of repro- 

 duction to another. The converse change could also be brought about. 



Certain writers have adopted the view that sex in animals 

 generally is regulated by the time at which fertilisation takes place, 

 that is to say, by the condition of the germ-cells. Thus, Thury 3 and 

 subsequently Busing 4 expressed the opinion that an egg which is 

 fertilised shortly after ovulation usually develops 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 oestrous periods, an early coitus being 

 supposed to result in the birth of a female, and a late coitus in the 

 production of a male. 



Pearl and Parshley/' who have dealt with the matter statistically 

 in the cattle at the Maine Experiment Station, have found that as 

 the time of copulation approaches the end of the osstrous period, 

 there is a progressive increase in the proportion of male calves born. 

 They have thus confirmed Thury and Diising. 



Pearl and Salaman have also investigated the question in man, 

 but found the evidence unsatisfactory. 



Richard Hertwig, 7 and later Kuschakewitsch, have carried out a 

 series of experiments on frogs, and these also show that over-ripeness 

 of the ova was associated with a preponderance of males, and in some 



1 Whitney, "The Control of Sex by Food in Five Species of Eotifers," 

 Jour, of E.rp. Zool., vol. xx., 1916. 



2 Shull, " Relative Effectiveness of Food, Oxygen, and other Substances in 

 Causing or Preventing Male Production in Hydatina" Jour, of E.rp. Zool., 

 vol. xxvi., 1918, and long series of papers on the same subject in the same 

 journal previous to this date. (See also below, p. 670.) 



3 Thury, Ueber das Gesetz dfr Erzeugung der (Jesc.hlechter, Leipzig, 1863. 



4 Diising, " Die Regulierung des Geschlechtsverhaltnisses bei der Vermeh- 

 rung, etc.," Jenaische Zeitsch., vol. xvii., 1884. 



6 Pearl and Parshley, "Data on Sex-determination in Cattle," Biol. Bull., 

 vol. xxiv., 1913. 



6 Pearl and Salaman, " The Relative Time of the Fertilisation of the Ovum 

 and the Sex-ratio amongst Jews," Amer. Anthropologist, vol. xv., 1913. 



7 Hertwig, " Ueber das Problem des sexuellen Differenzieurung," " Weitere 

 Untersuchungen, etc.," Verhaiull. Deutsch. Zool. (jes., vols. xv., xvi., and xvii., 

 1905-06-07. 



