528 LIFE : OUTLINES OF GENERAL BIOLOGY 



sex is determined when the primitive germ-cells divide into oocytes. 

 In his 1902 paper on The Determination of Sex in Animal 

 Development, Beard said: "Any interference with, or alteration of, 

 the determination of sex is absolutely beyond human power." To 

 this, an experimenter like Russo would answer that he has succeeded 

 in effectively interfering with the determination of sex. For although 

 it may not be possible to alter the bias of an egg which has become 

 fixed to develop into a male or into a female, it may be possible 

 by altered nutrition to change the numerical proportions of these 

 two kinds of eggs produced in the maternal ovary, and it may 

 be possible in other ways to change the normal proportions of 

 survival. 



Of great interest in connection with this third theory are the 

 facts of Polyembryony — the production of multiple embryos from 

 one ovum. Like "identical twins" the "polyembryonic" oifspring 



E^S 



Fig. 77. 



Polyembrj'ony of a Hymenoptcrous Insect (Encyrtus fuscicollis). After Marchal, 

 ' From one ovum there are developed numerous embryos (E), enveloped 

 in a ribbon of mucus (S). 



are always of the same sex. In one of the armadillos {Praopus or 

 Tatusia hyhrida) von Jhering found on two occasions eight embryos 

 within one chorion — presumably, therefore, from one ovum — and 

 all were males. In some of the parasitic Hymenoptcrous insects, 

 e.g. Encyrtus, investigated by Marchal and Bugnion, Litomastix 

 and Ageniaspis, investigated by Silvestri, one segmented ovum 

 forms a group of embryos, all of the same sex — female if the egg 

 be fertilised, male if it be not fertilised. These facts strongly confirm 

 the view that the sex of the offspring is already determined in 

 the egg. 



The theory has been more than once suggested that the ova 

 from one ovary develop into females and those from the other ovary 

 into males. Thus Dr. Rumley Dawson [The Causation of Sex, London, 

 1909) has maintained for man, that the ova produced by the right 

 ovary develop into males, and that those produced by the left 

 ovary develop into females. This view has been tested experimentally 

 in the rat by Doncaster and Marshall, who found that each rat, 



