142 GENERAL PRINCIPLES OF ZOOLOGY. 



(fig. 90), or if from the same mother numerous buds are simul- 

 taneously cut off (lateral and multiple budding) (compare fig. 20). 



1}. 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 thus acquire the capacity of 

 giving rise to a new organism. Now, since there are hermaph- 

 roditic animals which produce simultaneously eggs and sperma- 

 tozoa, and since with many of them at least the possibility of 

 self-fertilization has been demonstrated, it becomes clear that the 

 emphasis in the definition of sexual reproduction must be laid, not 

 upon the individual, but upon the 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 explanation is appli- 

 cable 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 thirty years it has been demonstrated in many 

 instances that two modes of reproduction formerly considered as 

 monogony, parthenogenesis and psedogenesis, must be regarded as 

 special modifications of sexual reproduction, although the above- 

 mentioned conditions are not strictly satisfied. In both cases the 

 eggs develop on account of some peculiar internal stimulus, 

 without the occurrence of fertilization ~by spermatozoa. In case of 

 pcedogenesis there is the additional circumstance 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 pupal stage and become flies. 

 Paedogenesis consequently is parthenogenesis in an immature 

 organism. 



Parthenogenesis and Typical Amphigony. Some have at- 

 tempted to exclude parthenogenesis from sexual reproduction by 

 claiming that those eggs which develop parthenogenetically are 

 pseudova, structures which are not actual eggs. This view is 

 absolutely untenable in view of the proof that the ' pseud ova' 

 arise just like ordinary eggs and develop like them, since they 

 cleave and form germ-layers. The equivalence of parthenogenetic 

 eggs to those which are fertilized is best shown in the case of the 

 bee, where similar cells give rise to a female or a male insect 



