Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote . elementarytextbo01clau Year: 1892-1893 124 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAX. Medusae). Accordingly we have to distinguish different forms of alternations of generations, which have genetically a different origin and explanation. The latter form of alternations of generations resembles metamor- phosis ; and we have in most cases to explain it as having arisen in the following way :—The asexual form corresponds to a lower stage in the phyl
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote . elementarytextbo01clau Year: 1892-1893 124 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAX. Medusae). Accordingly we have to distinguish different forms of alternations of generations, which have genetically a different origin and explanation. The latter form of alternations of generations resembles metamor- phosis ; and we have in most cases to explain it as having arisen in the following way :—The asexual form corresponds to a lower stage in the phylogenetic history, from which it has inherited the capacity of asexual reproduction, while the sexual reproduction belongs entirely to the higher form. To take as an example the alternation of generations of the Scyphomedusse. The animal is hatched as a free-swimming ciliated planula (gastrula with closed blastopore) (fig. 113 «). After a certain time it fixes itself by the pole of its body, d
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