Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 PIG. 110.—A, Unequal segmentation of the egg of Bonellia; £, epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streak is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an evolutio ex una parte, and the other an evolutio
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 PIG. 110.—A, Unequal segmentation of the egg of Bonellia; £, epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streak is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an evolutio ex una parte, and the other an evolutio ex omnibus partihus. It is not, however, possible to draw a sharp line between these two methods of development, nor have they the significance which was formerly ascribed to them, for closely allied forms may present great differences in this respect according to the amount of food yolk and the duration of the embryonic development. The Coelenterata, the Echinoderms, the lower Worms and Mol- luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) present us with examples of regular development of all parts of the
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