ZOOLOGICAL SCIENCE 191 



of development only after fertilization like that which 

 occurs in the great majority of animal forms. 



The natural parthenogenesis above described is a com- 

 paratively rare phenomenon. Its occurrence suggests that 

 eggs, which develop in nature only after fertilization, may 

 be caused to develop parthenogenetically if suitable stimuli 

 are applied. This is found to be the case; and the phenom- 

 enon is now designated artificial parthenogenesis, in contrast 

 to the natural parthenogenesis which occurs in nature. 

 Since the first successful experiments in artificial partheno- 

 genesis some twenty-five years ago, 4 it has been found that 

 the eggs of many animals, among which are worms, molluscs, 

 echinoderms, and vertebrates, may be thus caused to 

 develop without fertilization. Development, hardly to be 

 distinguished from that which is normal, ensues when these 

 eggs are subjected to very dilute solutions of salts, acids, 

 narcotics, and other substances, to changes in temperature 

 and in some cases even to simple mechanical stimulation. 

 It is quite conceivable that there is no egg of any animal 

 which could not be artificially started on its development by 

 the application of a suitable stimulus. 5 



Experiments such as these give an understanding of fer- 

 tilization which could never be obtained by mere observation 



4 The first recorded attempts at artificial parthenogenesis are those of 

 Spallanzani (1785), who attempted "to start the development of eggs by 

 electricity, by the action of extracts of all the various organs, by vinegar, 

 dilute alcohol, lemon juice and other substances, all without effect." Lillie, 

 F. R., "The History of the Fertilization Problem," loc. tit. 



5 As yet it has been impossible to carry through to an adult state the embryos 

 thus formed, save in a few exceptional cases. But this could hardly be ex- 

 pected at the present stage of investigation, because the initiation of develop- 

 ment, by these artificial means, is so wide a departure from the normal process. 

 Moreover, the normally fertilized eggs of these forms are reared with difficulty 

 in the laboratory. But there is a reasonable expectation that once the tech- 

 nique is discovered many artificially fertilizable eggs may be carried through 

 their entire cycle to the adult. Should this be accomplished, it would throw 

 light upon problems of sex and of heredity, because the adults thus formed 

 would be without male parentage in the generation which immediately 

 ceded them. 



