GENERAL EMBRYOLOGY 



129 



b. Sexual Reproduction: AmpJiigony. 



Amphigony. — 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 hermaphroditic animals and since with many of them 

 the possibility of self-fertilization has been demonstrated, it becomes clear 



Fig. 93. — -4 , Hydra grisea with a bud ; B, first stage of bud. en, entoderm; ec, ectoderm; 

 s, supporting lamella; I, tentacle of mother and bud; m, stomach; 0, mouth. 



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 reproduetion is to lie sought in the union of male and 

 female sexual cells. 



Parthenogenesis and Paedogenesis.- — This explanation is applicable 

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

 term sexual reproduction can be applied. Still, it has been demonstrated 

 in many instances that two modes of reproduction formerly considered as 

 monogony — parthenogenesis and pasdogenesis — must be regarded as 

 modifications of sexual reproduction, although the conditions mentioned 

 above are not strictly satisfied. In both cases the eggs develop be- 

 cause of some peculiar internal stimulus, without the occiirrence of 

 fertilization by spermatozoa. In case of pccdogenesis there is the addi- 

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