. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. DilillCr DKVELOPilEJfX AKD METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endodorm of the hypothe- tical Gastrrea; while for the middle layer, wliich is only secondaiily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this tlieory, which is esi^entially an extension of the Baer-llemak theory of the germinal layers from the Vertebrata to the who
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. DilillCr DKVELOPilEJfX AKD METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endodorm of the hypothe- tical Gastrrea; while for the middle layer, wliich is only secondaiily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this tlieory, which is esi^entially an extension of the Baer-llemak theory of the germinal layers from the Vertebrata to the whole group of Metazoa, with its pretentious and hasty speculation has created a basis for comparative embryology; such a basis can only be obtained as the result of comprehensive investigations. DIRECT DEVELOPMENT AND METAMORPHOSIS. Tlie more complete the agreement between the just born young and the adult sexual animal, so much the greater, especially in the higher animals, will be the du ration of the embryonic development and the more complicated the developmental processes of the embryo. The post-embryonic develop- ment will, in this case, be confined to simple processes of growth and perfection of the sexual organs. When, how- ever, embryonic life has, relatively to the height of the organization, a quick and simple course ; when, in other words, the embryo is born in an immature condition and at a relatively low stage of organization, the post-embryonic development will be more complicated, and the young animal, in addition to its increase in size, will present various processes of transformation and change of form. In such cases, the just hatched young, as opposed to the adult animal, is called a Larva, and develops gradually to the form of the aduK-. Fig. 111.—Larval stages of the Frog (after Eckev). a, embryo some time before hatcliing, with wart-like gill papilUe (m the visceral arches, b. Larva some time after hatching, with external branchiae, c, Cider larva, with h
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
