THE BREEDING SEASON 7 



composition of the medium surrounding the culture. 1 (See 

 p. 213.) 



Moreover, there is evidence that in the case of Colpoda 

 steini at least the occurrence of conjugation is determined 

 entirely by the conditions of the surrounding medium. 



CCELENTERATA 



With the majority of the Metazoa, as already indicated, 

 there is a more or less definitely restricted season to which the 

 occurrence of the chief reproductive processes is confined. 



Thus in the common hydra of Bengal (Hydra orientalis, 

 Annandale), which, like most other Ccelenterates, reproduces 

 by budding as well as by the sexual method, 2 the former process 

 occurs chiefly 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 environ- 

 ment 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 fol- 

 lowed by a sudden but not excessive rise of temperature. 3 



1 Calkins, loc. cit. 



2 Asexual reproduction is of very common occurrence among the majority 

 of the lower animals and plants. It may take the form of simple binary 

 fission (in unicellular organisms), of spore formation, or of germination or 

 budding. Sexual reproduction consists essentially of the union of two cells 

 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 Chap. VI.) 



3 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 viridis," 



