THE BREEDING SEASON 7 



during winter, the buds developing into new individuals. Towards 

 the beginning of the hot weather budding becomes less active, 

 and in some individuals ceases altogether, while the same thing 

 happens during periods of temporary warmth in winter. A rise in 

 temperature induces a proportion of the individuals present in an 

 aquarium or pond to develop testes or male reproductive glands; 

 if the rise is considerable it may cause a few of the remaining 

 individuals to produce ova. On the other hand, no individual living 

 in its natural environment has been known to exhibit any sign of 

 sex after the rise in temperature had become steady. The conditions 

 most favourable to the production of ova appear to be a period of 

 comparatively low temperature and abundant nutrition followed by 

 a sudden but not excessive rise of temperature. 1 



Some of the marine hydroids show an alternation of generations 

 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 interpretation is correct the breeding season among marine 

 hydroids is controlled by environmental conditions, just as it is 

 among most other animals. 2 



and their subsequent division to give rise to the new individual. In the 

 multicellular organisms (Metazoa and Metaphyta) there are two kinds of con- 

 jugating cells, or gametes, which are specialised for the purpose. These are 

 produced by the male and female respectively, and are known as spermatozoa 

 and ova. Thus, sexual reproduction in the Metazoa is a modification of con- 

 jugation in the Protozoa. (See Chapter VI.) 



1 Annandale, "The Common Hydra of Bengal," Memoirs of the Asiatic 

 Society of Bengal, vol. i., 1906. Of, Whitney, "The Influence of External 

 Factors in causing the Development of the Sexual Organs in Hydra mridis" 

 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. 



2 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 being 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 



