Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 fis 100 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-yascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common to the ova and spermatozoa (fig. 92), but which, as in
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 fis 100 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-yascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common to the ova and spermatozoa (fig. 92), but which, as in Helix (fig. 93), may partially separate into vas deferens and oviduct. In other CUM'. the ovaries and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trernatocln, rhabdoccele Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appears to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertilization is sufficient for the production of oil- spring. But this original condition of self-fertiliza- tion appears to be the ex- ception in almost all hermaphrodites. In those animals in which the ovary and testis are not com- pletely separated from one another cross-fertilization is rendered necessary, and self-fertilization prevented by the fact that the male and female elements are matured at different times (Snails*Salps). From this form of complete hermaphroditism the generative organs pass through a stage of incomplete hermaphroditism, in which, though the organs of both sexes are present, one of them is rudi- mentary, to reach the dioecious condition in which the sexes are completely separated (Distonmmfitticolle and hce-matobium). Animals in which the sexes are distinct not unfrequently present traces of an FIG. Sexual organs of the Roman Snail (Helix pomatia). Z
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