. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. DilUXT DJSVELOPilEKT AND METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endoderm of the hypothe- tical Gastrtea; 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 Vertebrata to the
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. DilUXT DJSVELOPilEKT AND METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endoderm of the hypothe- tical Gastrtea; 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 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. 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 ; 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 acini*--. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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