. Elementary text-book of zoology. Zoology. DEVELOPMENT. 185 alone have copulatory organs in the form of the swollen lips of the cloaca, which during copulation clasp the cloacal aperture of tin- female, and thus render an internal fertilization possible. In this the eggs can undergo their development within the body of the female, and the young be born at a more or less advanced stage of development. It is only in exceptional cases that the parents have an instinct which leads them to watch over the further fate of their brood, as for example Alijtes (fig. 626) and the South American Su
. Elementary text-book of zoology. Zoology. DEVELOPMENT. 185 alone have copulatory organs in the form of the swollen lips of the cloaca, which during copulation clasp the cloacal aperture of tin- female, and thus render an internal fertilization possible. In this the eggs can undergo their development within the body of the female, and the young be born at a more or less advanced stage of development. It is only in exceptional cases that the parents have an instinct which leads them to watch over the further fate of their brood, as for example Alijtes (fig. 626) and the South American Surinam Toad (Pipa dorsigera). The male of Alytes winds the string of eggs round its hind legs and burrows into the damp earth, and only gets rid of his load when the embryonic development is completed. The male of Pipa places the eggs when laid on the back of the female, which then develops a cell-like pouch round each egg. The larva? are hatched and undergo their metamorphosis in these pouches. In other genera, as Notodelphys, the females possess a spacious brood-sac beneath the dor- sal integument. Except in these cases, the eggs are either attached singly to water plants (TritonidcB\ or laid in strings or irregular clumps. Development.—The eggs, which are rela- tively small,* undergo an unequal segmenta- tion (vol. i., fig. 104) after fertilization. The Amphibia agree with the Fishes in not developing an amnion or allantois — the embryonic membranes of such importance in the higher Verte- brates. In the Amphibians, however, the urinary bladder which arises from the ventral wall of the cloaca is morphologically equiva- lent to the allantois. The embryos are also without any external yolk-sac constricted off from the body, the yolk being enclosed at an early period by the ventral plates. As respiratory organs gills are. FIG. 620.—Alytm obstetricant. Male with the string of eggs. * ('. E. v. Baer, '• Ueber die Entwickelungsgeschichte der Thiere,'' Kiinigsbeiy. 1837.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
