The physiology of reproduction / by Francis H.A. Marshall with a preface by Professor E.A. Schäfer ... and contributions by William Cramer ... and James Lochhead.

  • Marshall, F. H. A. (Francis Hugh Adam), 1878-1949.
Date:
1910
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Licence: Public Domain Mark

Credit: The physiology of reproduction / by Francis H.A. Marshall with a preface by Professor E.A. Schäfer ... and contributions by William Cramer ... and James Lochhead. Source: Wellcome Collection.

    32/736 (page 8)
    Some of the marine hydroids show an alternation of genera- tions which does not appear at first sight to be in any way related to change in the environment. In such cases the fertilised ovum develops into a polyp which gives rise to a colony of polyps by a process of sexual reproduction. After the colony has reached a certain size, a new kind of bud is formed, and this becomes a jelly-fish. The latter, after leading an independent existence, produces eggs, and these in turn become fertilised, giving rise to a new generation of polyps. Morgan points out that as the polyp colony goes on increasing in size, its relation to its surroundings must undergo change, and that, very possibly, it is this change which determines the development of jelly-fish in place of polyps. If this inter- pretation is correct the breeding season among marine hydroids is controlled by environmental conditions, just as it is among most other animals. Some interesting observations have been recorded by Ashworth and Annandale? about the breeding habits of sea- anemones. The species Sagartia troglodytes and Actinia mesem- bryanthemum, which are very prolific in captivity, have been noticed to breed regularly in the early spring. Actinva com- mences to produce young in the beginning of February, and Sagartia about a month later. As a rule the young are ex- truded in the early morning, and one individual may repeat Arch. f. Entwick.-Mechanik, vol. xxiv., 1907. Whitney says that in Hydra viridis an abundance of food following a low temperature causes a suppression of the formation of testes and ova. 1 Morgan, Experimental Zoology, New York, 1907. Morgan shows that the same point is illustrated by certain recent experiments of Klebs on flowering plants. These at first produce only leaves and branches. When they reach a certain size they produce flowers. Klebs regards the develop- ment of the flowers as beiug due to a relation that becomes established between the plant (when it has reached a certain stage of growth) and the environment. He shows also that by altering the environment a shoot may be induced to go on growing vegetatively, when it would ordinarily develop into a flowering branch. ‘The flowering of the plant, therefore, is not merely the culmination of its form, as most botanists regard it. For much valuable and suggestive information on the factors which control breeding in plants G. Klebs’ work should be consulted. ( Willkiirliche Entwickelungstnderungen bei Pflanzen, 1903.) 2 Ashworth and Annandale, ‘‘Observations on some Aged Specimens of Sagartia troglodytes, and on the Duration of Life in Ccelenterates,’’ Proc, Roy. Soc. Edin., vol. xxv., 1904.