x] SEX 151 



which under suitable conditions can take place equally well 

 without it. It is possible, however, to some extent to re- 

 verse the parts, for a spermatozoon may enter a non- 

 nucleated portion of an egg and give rise to a nucleus which 

 divides and causes the piece of egg to segment and develop 

 into a larva. In such a case there is something of the nature 

 of male parthenogenesis, for although the cytoplasm is of 

 maternal origin, the whole of the nuclear matter is descended 

 from the spermatozoon only. 



A number of problems arising from artificial partheno- 

 genesis are still unsolved. One of the most interesting is 

 concerned with the determination of sex, which will be 

 further dealt with in the next chapter. If, as now seems to 

 be almost certain, the sex of every individual depends on 

 the presence and activity of certain chromosomes, important 

 experimental evidence should be obtainable from animals 

 reared to maturity from unfertilised eggs, but unfortunately 

 few animals have yet been reared in this way to a stage at 

 which the sex could be decided with certainty. LOEB (1918), 

 however, finds that parthenogenetically-produced frogs may 

 be either male or female; out of nine individuals reared to 

 an age at which the sex could be distinguished with cer- 

 tainty seven were male and two female. It appeared that 

 the males had the diploid chromosome number in the 

 spermatogonial nuclei. 



