260 ANIMAL BIOLOGY 



consider some evidence among higher forms in regard to the 

 dynamic influence of fertilization. Although fertilization is usu- 

 ally necessary for the resumption of the series of cell divisions 

 which paused after the maturation divisions, and which are to 

 transform egg into adult, there are many exceptional but entirely 

 normal cases where the egg proceeds to divide of its own accord. 

 Such parthenogenetic eggs are formed like other eggs, though 

 sometimes without chromosome reduction. Thus the eggs of the 

 Honey Bee, to cite the most interesting case, develop either with 

 or without fertilization - - fertilized eggs forming females and un- 

 fertilized eggs, males. Certain species of Rotifers and Roundworms 

 apparently reproduce solely by parthenogenesis, males not being 

 known. Leaving out of the question the effect on the chromosome 

 complex, it is at once apparent that the mere fact that an egg di- 

 vides without the influence of a sperm indicates clearly that, in such 

 cases at least, neither structural additions nor physiological in- 

 fluences of the sperm are necessary to initiate development. 



It may with justice be urged, however, that such cases of normal 

 parthenogenesis are special adaptations to peculiar conditions in 

 which the egg has usurped, as it were, the usual sperm function, 

 and that therefore the evidence is of little weight in determining 

 the primary significance of fertilization. Accordingly the data 

 from so-called artificial parthenogenesis are particularly co- 

 gent. Within recent years it has been found that the eggs of a con- 

 siderable number of Invertebrates and even of Vertebrates, such 

 as some Fishes and Frogs, which normally require fertilization, 

 can be induced to start development parthenogenetically by var- 

 ious artificial means such as subjection to certain chemicals, 

 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 con- 

 clusively that even eggs which normally require fertilization are 

 intrinsically self-sufficient at least to start to develop, and there- 

 fore this strongly indicates that an incidental and not the main 

 function of fertilization is to stimulate cell division. 



Restating the evidence in its bearings on the meaning of fertili- 

 zation, we may say that fertilization is not fundamentally an 



