. Elementary text-book of zoology. Zoology. DIRECT DEVELOPMENT AND METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa; the one being traced bu-k to the ectoderm and the other to the endoderm of the hypothe- tical Gastnea; while for the middle layer, which is only secondarily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this theory, which is essentially an extension of the Baer-Remak theory of the germinal layers from the Yertebrata to the whole group of Metazoa, with its pretentious and hasty speculatio
. Elementary text-book of zoology. Zoology. DIRECT DEVELOPMENT AND METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa; the one being traced bu-k to the ectoderm and the other to the endoderm of the hypothe- tical Gastnea; while for the middle layer, which is only secondarily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this theory, which is essentially an extension of the Baer-Remak theory of the germinal layers from the Yertebrata 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. The 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; FlG stages of the Frog (after Ecker) „ when, ill Other Words, embryo some time before hatching-, with wart-like gill , •, , . -I papillae on the visceral arches. I, Larva some time tne embryo IS born in after hatching, with external branchite. c, Older larva, an immature condition with norny beak and small branchial clefts beneath the integumentary operculum, with internal branchia? • and at a relatively low N, nasal pit; S, sucker; E, branchise; A, eye; II:, stage of organization, horny teeth. 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
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
