ORIGIN OF THE INDIVIDUAL 243 



tilization a phenomenon associated with reproduction so 

 closely associated in nearly all organisms that the two pro- 

 cesses, evolving together from relative simplicity to great 

 complexity, have reciprocally influenced one another until in 

 higher forms they seem to be related as cause and effect, and 

 reproduction becomes dependent on fertilization. With this 

 somewhat didactic statement of our viewpoint, we may con- 

 sider some of the salient features of the almost endless discus- 

 sion of the significance of fertilization. 



Quite naturally the original view, emphasized by Harvey 

 and a long series of successors, was that fertilization funda- 

 mentally is a reproductive process, and echoes of this idea 

 are preserved in certain present-day hypotheses, on the basis 

 of such facts as the following. The mature egg pauses in de- 

 velopment and usually comes to naught unless fertilized 

 the entrance of the sperm affording a necessary stimulus for 

 the resumption of cell division which is to transform the egg 

 into the adult. . Again, the egg typically contains only half 

 the somatic chromosome complex (simplex group, haploid 

 number) and most of the cytoplasm, while the sperm con- 

 tributes a reciprocal haploid set of chromosomes; in short, 

 seemingly transforms what is essentially a half into a whole. 



However, it does not necessarily follow from these facts 

 that fertilization is primarily a reproductive process. The 

 evidence against this conclusion is derived largely from the 

 relations of fertilization and reproduction in the Protista, and 

 trom the development, in certain cases, of eggs without 

 fertilization, or by PARTHENOGENESIS. A single example of 

 each class of facts will suffice. 



1 . Protista 



The life histories of nearly all Protozoa and Protophyta 

 which have been carefully studied include a period in which 



