350 CLASSICS OF MODERN SCIENCE 

 among themselves in the molecular structure of their nucleoplasm. 

 Early in development each cell must possess its ovi^n peculiar nucleo- 

 plasm, for the further course of development is peculiar to each 

 cell. It is only in the later stages that equivalent or nearly equivalent 

 cells are formed in large numbers, cells in which we must also sup- 

 pose the existence of equivalent nucleoplasm. 



If we may assume that a certain amount of germ-plasm must be 

 contained in the segmentation nucleus in order to complete the whole 

 process of the ontogenetic differentiation of this substance; if we may 

 further assume that the quantity of germ-plasm in the segmentation 

 nucleus varies in different cases ; then we should be able to understand 

 why one egg can only develop after fertilization, while another can 

 begin its development without fertilization, but cannot finish it, and 

 why a third is even able to complete its development. We should 

 also understand why one egg only passes through the first stages of 

 segmentation and is then arrested, while another reaches a few more 

 stages in advance, and a third develops so far that the embryo is 

 nearly completely formed. These differences would depend upon the 

 extent to which the germ-plasm, originally present in the egg, was 

 sufficient for the development of the latter; development will be ar- 

 rested as soon as the nucleoplasm is no longer capable of producing 

 the succeeding stage, and is thus unable to enter upon the following 

 nuclear division. 



From a general point of view such a theory would explain many 

 difficulties, and it would render possible an explanation of the phyletic 

 origin of parthenogenesis, and an adequate understanding of the 

 strange and often apparently abrupt and arbitrary manner of its oc- 

 currence. In my works on Daphnidae I have already laid especial 

 stress upon the proposition that parthenogenesis in insects and Crusta- 

 cea certainly cannot be an ancestral condition which has been 

 transm.itted by heredity, but that it has been derived from a sexual 

 condition. In what other way can we explain the fact that partheno- 

 genesis is present in certain species or genera, but absent in others 

 closely allied to them ; or the fact that males are entirely wanting in 

 species of which the females possess a complete apparatus for fertili- 

 zation? I will not repeat all the arguments with which I attempted to 

 support this conclusion. Such a conclusion may be almost certainly 

 accepted for the Daphnidae, because parthenogenesis does not occur 



