1 70 GENERAL PRINCIPLES OF ZOOLOGY. 



b. Sexual Reproduction: Amphigony. 



Amphigony Defined. For sexual reproduction two 

 animals are commonly necessary, a female and a male ; the 

 reproductive cells, the eggs, of one must be fertilized by 

 the reproductive cells, the spermatozoa, of the other, and 

 thereby acquire the capacity of giving rise to a new or- 

 ganism. Now, since there are hermaphroditic animals 

 which produce simultaneously eggs and spermatozoa, and 

 since many of them at least lack the power of self-fer- 

 tilization, it becomes clear that the emphasis in the defi- 

 nition of sexual reproduction must be laid, not upon the 

 individual, but upon their sexual products. Consequently 

 the essential point of sexual reproduction is to be sought in 

 the union of male and female sexual cells. 



Parthenogenesis and Paedogenesis. This explana- 

 tion is applicable to by far the greater majority of cases, 

 namely, to all cases where the term " sexual reproduction " 

 can be applied. Still, in the course of the last twenty years 

 it has been demonstrated in many instances that two modes 

 of reproduction, formerly considered as monogony, parthe- 

 nogenesis and pcedogenesis, must be regarded as special 

 modifications of sexual reproduction, although the above- 

 mentioned conditions are not strictly fulfilled. In both 

 cases the eggs develop on account of some peculiar internal 

 stimulus, without the occurrence of fertilization by sperma- 

 tozoa. In case of pcedogcncsis there is the additional cir- 

 cumstance that reproduction is accomplished by animals 

 which have not completed their normal development ; for 

 example, the larvae of certain flies reproduce before they 

 have passed through the pupa stage and become flies. 

 Pcedogenesis consequently is the parthenogenesis of an im- 



mature organism. 



Parthenogenesis and Typical Amphigony. Some 

 investigators have attempted to exclude parthenogenesis 

 from sexual reproduction, by claiming that those eggs 

 which develop parthenogenetically are pseud-ova, structures 



