PROCESSES RELATED TO SEXUAL REPRODUCTION 235 



union of germplasms, or amphimixis, which is the basis of bi- 

 parental inheritance, and the activation by which the egg is 

 stimulated to develop instead of dying as it would do eventually 

 if unfertilized. 



Natural and Artificial Parthenogenesis. — In this connection, 

 the phenomenon of -parthenogenesis, in which an ovum develops 

 without the entrance of a spermatozoon, may be described. Such 

 a process occurs normally in a considerable number of the Arthro- 

 poda (c/. p. 241), hke insects and spiders, in some Platyhelminthes 

 and Trochelminthes, and perhaps in some of the lower Verte- 

 brata. Males are known to exist in most of these cases, and 

 fertilization of the eggs occurs in certain generations; or certain eggs 

 may be fertilized while others are not, as in the honey bee. Where 

 males are unknown, it is presumed that the}' have not yet been 

 discovered, although it would be theoretically possible to have the 

 male sex completely eliminated in the evolution of a species with 

 such a mode of reproduction. Hence, it appears that species 

 exhibiting natural parthenogenesis produce, under certain condi- 

 tions, eggs that develop without fertilization and other eggs that 

 develop only when normally fertihzed as in the great majority of 

 animals. 



The occurrence cf natural parthenogenesis suggests that eggs 

 that develop in nature only after normal fortiUzation may be 

 caused to develop parthenogenetically if suitable stimuli can be 

 applied. This is found to be the case; and the phenomenon is 

 known as artificial parthenogenesis in contrast to the partheno- 

 genesis that occurs under natural conditions. Since the first suc- 

 cessful experiments in artificial parthenogenesis were conducted, 

 about 1900, it has been found that the eggs of many animals, 

 among which are worms, molluscs, echinoderms, and vertebrates, 

 may be caused to develop in the absence of fertilization. Develop- 

 ment, which in some few cases has progressed to a late stage, 

 ensues when such eggs are subjected to very dilute solutions of 

 salts, acids, narcotics, and other substances, to changes in temper- 

 ature, and in some instances even to simple mechanical stimula- 

 tion. It is conceivable that there is no egg of any animal that 

 might not be thus " artificially " started on its development by the 

 application of a suitable stimulus. 



The reproductive processes of animals, therefore, have certain 

 fundamental resemblances. This is particularly true of sexual 



