250 FOUNDATIONS OF BIOLOGY 



various artificial means such as subjection to certain chemi- 

 cals, unusual temperature changes, shaking, or the prick of a 

 needle the effective stimulus varying with different species. 



Just what happens in the egg as a result of such treatment 

 is open to discussion, but for our purposes it is sufficient to 

 know that the egg begins to divide in normal fashion. This 

 shows conclusively that even eggs which normally require 

 fertilization are intrinsically self-sufficient at least to start to 

 develop, and therefore this strongly indicates that an inci- 

 dental and not the main function of fertilization is to stimu- 

 late cell division. 



Restating the evidence in its bearings on the meaning of 

 fertilization, we may say that conjugation, under suitable 

 environmental conditions, is not fundamentally an indis- 

 pensable event in the life history of the Protozoa, and further 

 that whatever stimulus is associated with fertilization is 

 also provided by endomixis which does not involve synkaryon 

 formation. Similarly in the Metazoa, both normal and arti- 

 ficial parthenogenesis indicate that the egg itself comprises 

 a mechanism which is capable of initiating and carrying on 

 development. From this viewpoint, fertilization may be 

 satisfactorily interpreted as a means of insuring under special 

 or unfavorable environmental conditions in unicellular or- 

 ganisms, and under usual conditions in the eggs of multicellu- 

 lar forms, a suitable stimulus which otherwise might be un 

 available at the proper time. 



Granting then that one aspect of fertilization is 'dynamic,' 

 what is its main significance? Many lines of evidence at 

 present are slowly but surely converging toward the view that 

 the opportunities which fertilization affords for changes in 

 the complex of the germ are of paramount importance. 

 Fertilization establishes new duplex groups of hereditary 

 characters by combining diverse simplex groups from the 



