Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 130 ORGANIZATION AND DEVELOPMENT OF ANIMALS. It is, however, extremely probable that the masses of cells from which the Cercariae arise represent the rudiments of ovaries, the elements of which develop parthenogenetically without the addition of spermatozoa. The so-called germ sacs (Sporocysts or Rediae) would in this case be larvae, which possess the power of reproduction; and the development of- the Distomeaa would conie under the head of heterogamy. The Cercarise, however, represent a
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 130 ORGANIZATION AND DEVELOPMENT OF ANIMALS. It is, however, extremely probable that the masses of cells from which the Cercariae arise represent the rudiments of ovaries, the elements of which develop parthenogenetically without the addition of spermatozoa. The so-called germ sacs (Sporocysts or Rediae) would in this case be larvae, which possess the power of reproduction; and the development of- the Distomeaa would conie under the head of heterogamy. The Cercarise, however, represent a second and more advanced larval phase. Provided with a motile tail, frequently with eyes and buccal spine, their organization, save in the absence of developed generative organs, presents great simi- larities to the sexually mature adults into which they develop. This development, however, takes place only in the body of another and usually more highly organized host after the loss of the larval organs. If the conception of a spore as an asexual reproductive product be main- tained, it becomes impossible in practice to draw a sharp line between alterna- tion of generations and heterogamy; since there is no test which enables us to distinguish between a spore and an ovum which develops parthenogeneti- cally. On the other hand, if we inter- pret, as we are justified in doing, the so-called spores as precociously developed ova, alternation of generations and heterogamy can be clearly distinguished from one another, since in the former one generation is asexual, and in- creases entirely by budding and division; while in the latter both generations are sexual, though in one of them the ova may possess the power of spontaneous development. An essential characteristic both of heterogamy and alternation of generations depends upon the different form of the individuals appealing in the generations which usually occur in a regularly alternating manner in the life-history of the species.
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