490 HEREDITY AND SEX 



In some of the higher Pteridophytes there are two kinds 

 of spores, micro- and macro-spores, which produce respectively 

 male and female prothallia. Prof. E. B. Wilson notes that 

 a similar predestination, not marked by visible differences, has 

 been proved by Blakeslee in both zygotes and spores of various 

 species of fungi, and that it has also been demonstrated in liver- 

 worts and mosses. He refers in particular to the recent studies 

 of the Marchals on dioecious mosses. " Isolation cultures prove 

 that the asexual spores, though similar in appearance, are in- 

 dividually predestined as male-producing and female-producing ; 

 and all efforts to alter this predestination by changes in the 

 conditions of nutrition, such as are known to be effective in the 

 case of fern prothallia, failed to produce the least effect." 



The view that there are two kinds of ova, determined ab 

 initio as male-producers and female-producers, has a vigorous 

 supporter in Beard, who finds evidence in the skate. He main- 

 tains that the sex is determined when the primitive germ-cells 

 divide into oocytes. In his 1902 paper on *' The Determination 

 of Sex in Animal Development," Beard scouted the idea of en- 

 vironmental interference with the determination of sex. " Any 

 interference with, or alteration of, the determination of sex is 

 absolutely beyond human power. To hope ever to influence or 

 modify its manifestations would be not less futile and vain than 

 to imagine it possible for man to breathe the breath of life into 

 inanimate matter." To this, an experimenter like Russo would 

 answer that he has succeeded in effectively interfering with the 

 determination of sex. Although it may not be possible to alter 

 the bias of an egg which has become fixed as a male-producer 

 or a female-producer, it may be possible by altered nutrition to 

 change the proportions of these two kinds of eggs 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 the third theory are 

 the facts of poly-embryony— the production of multiple embryos 





