. A manual of zoology. Zoology. 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;
. A manual of zoology. Zoology. 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 —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- 9. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929, ed. and tr. New York, H. Holt
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