50 SCIENCE PROGRESS 



gressed as far as the splitting of the chromosomes, and then 

 the spindle has broken down and the nucleus returned to the 

 resting condition. At the next division after such a process the 

 chromosome number would of course be found to be doubled, 

 and it is not unlikely that it is from the observation of cases 

 such as this that the statements about the restoration of the 

 normal number have been made. 



These facts lead up to one very interesting conclusion, which 

 could not have been reached apart from the study of partheno- 

 genesis, namely, that half the normal number of chromosomes is 

 sufficient for the life and growth of the individual. It has been 

 supposed that in an individual derived from a fertilised egg each 

 nucleus contains a double set of chromosomes, one derived from 

 the egg and the other from the spermatozoon, but that if one 

 complete set is present it will contain all the qualities necessary 

 for the production of an individual. But Boveri 1 has shown 

 that if the complete set is not present, even when the nucleus 

 includes more than the necessary number of chromosomes, the 

 egg will not produce a complete individual. This, as he points 

 out, is very strong evidence in favour of the individuality and 

 difference of function of different chromosomes. 



Another cell-organ on which parthenogenesis sheds some 

 light is the centrosome. In fertilised eggs it appears to arise 

 from or in connection with the spermatozoon, but as it exists 

 in the cells derived from virgin eggs it is clear that it can arise 

 de novo and that the spermatozoon is not required for its 

 production. 



Probably the two most interesting subjects on which the 

 study of parthenogenesis may shed light are those of variation 

 and the determination of sex. 



Weismann supposed that variation was largely caused by 

 the conjugation of somewhat unlike nuclei in fertilisation, and 

 this hypothesis has led to statements that parthenogenetic 

 animals do not vary, or do so less than those which are sexually 

 produced. Measurements of such forms as Daphnia and 

 Aphis 2 have not confirmed this idea, but show that varia- 

 bility is not widely different in parthenogenetic and sexual 

 species. And that such a conclusion was to be expected 

 might have been inferred from the polymorphism of partheno- 



1 Ergebnisse iiber die Konstitution, etc., des Zellkerns, 1904 (Fischer). 



2 Warren, Biometrika, i. 1907, p. 129. 



