8 THE PHYSIOLOGY OF REPRODUCTION 



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. 1 



Some interesting observations have been recorded by 

 Ash worth and Annandale 2 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. Actinia 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. ( Willkilrliche Entwickelungsanderungcn 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. 



