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). 



I). 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 Ijy the rei^roductive cells — 

 the spermatozoa — of the other, and thus acquire the capacity of 

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

 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 iu the definition of sexual rej)roduction 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 exjilanation is appli- 

 cable to by far the greater majority of cases, namely, to all cases 

 where the term sexual reproduction can be ajiplied. 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 pasdogenesis, must be regarded as 

 special modifications of sexual rej)roduction, although the above- 

 mentioned conditions are not strictly satisfied. In both cases the 

 eggs develop on account of some peculiar internal stimulus, 

 without tlie occurrence of fertilization hy spermaiozoa. In case of 

 ficedorjenesis there is the additional circumstance that reproduction 

 is accomplished by animals which have not completed their normal 

 development; for example, the larvas of certain flies reproduce 

 before they have passed through the pupal stage and become flies. 

 Pedogenesis 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 parthenogeuetieally are 

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

 absolutely untenable in view of the proof that the 'pseudova" 

 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 



