VI.] THETR SIGNIFICANCE IN HEREDITY. 357 



parthenogenetic as in the sexual ^gg. I had, however, been 

 mistaken in a matter of detail ; for the difference in the 

 quantities of nuclear substance is not produced by the ex- 

 pulsion of two polar bodies, and the reduction of the nuclear 

 substance to a quarter of its original amount, in both eggs, 

 while the parthenogenetic ^gg then doubles its nuclear sub- 

 stance b}^ growth ; but the difference is produced because the 

 reduction of nuclear substance originally present is less in one 

 case than it is in the other. In the parthenogenetic Qgg the 

 nuclear substance is only reduced to one-half by a single 

 division ; in the sexual ^gg it is reduced to a quarter by two 

 successive divisions. It is an obvious conclusion from this fact, 

 if proved to be wide-spread, that the significance of the first 

 polar body must be different from that of the second. Onh' one 

 polar body can signify the removal of ovogenetic nucleoplasm 

 from the mature ^gg^ and the second is obviously a reduction 

 of the germ-plasm itself to half of its original amount. This 

 very point seemed to me to be of great importance, because, 

 as I had foreseen long ago, and as will be shown later on, the 

 theory of heredity forces us to suppose that every fertilization 

 must be preceded by a reduction of the ancestral idioplasms 

 present in the nucleus of the parent germ-cell, to one-half of 

 their former number. 



But before the full bearing of the phenomena could be con- 

 sidered, it was necessary to ascertain how far they were of 

 general occurrence. There were two waj'^s in which this might 

 be achieved, and in which it was possible to prove that par- 

 thenogenetic eggs expel only one polar body, while sexual 

 eggs expel two. We might attempt to observe the phenomena 

 of maturation in both kinds of eggs in a species which repro- 

 duces itself by the parthenogenetic as well as the sexual 

 method. This would be the simplest way in which the 

 question could be decided, if it were possible to make such 

 observations on a sufficient number of species. But the other 

 method was also open, a method which would have been the 

 only one, if we did not know of any animals with two kinds of 

 reproduction. We might attempt to investigate the phenomena 

 of maturation in a large number of parthenogenetic eggs, if 

 possible from different groups of animals, and we might com- 

 pare the results with the facts which are already certain 



