6 THE PHYSIOLOGY OF REPRODUCTION 



been reached in the life-cycle, than upon the influences of the 

 environment. Thus, there is a period of extreme vigour of cell- 

 multiplication, corresponding to the youth of a metazoon ; secondly, 

 there is a period of maturity, characterised by changes in the 

 chemical and physical properties of the cell, and leading to the 

 formation of conjugating individuals ; and finally, in forms which 

 do not conjugate, there is a period of senescence which ends in 

 death. It is interesting to note, however, that the rapidity of 

 fission is affected by the temperature and the food; for example, 

 an individual of the Ciliate Infusoriau, Stylonychia pustulata, if 

 well supplied with food, divides once in twenty-four hours in a 

 temperature of from 5 to 10 (1, and once in twelve hours in a 

 temperature of from 10 to 15 C. 1 In Paramcecium aurelia, too, 

 it has been found that the rate of reproduction is influenced by 

 temperature after the manner of a chemical reaction. 2 Again, 

 Flagellate Infusoria of different kinds have been induced to conjugate 

 by changing the temperature or increasing the density in the 

 surrounding medium. 3 Furthermore, the life-cycle of Paramcecium 

 may be renewed without the occurrence of conjugation, that is to 

 say, fission can be made to continue and senescence can be 

 avoided, by introducing a change in the composition of the medium 

 surrounding the culture. 4 (See p. 222.) 



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 wientalis, Annan- 

 dale), which, like most other Ccelenterates, reproduces by budding 

 as well as by the sexual method, 5 the former process occurs chiefly 



1 Sedgwick, Student's Text-Book of Zoology, vol. i., London, 1898. 



2 Woodruff and Britsell, "The Temperature Coefficient of the Bate of 

 Reproduction of Paramcecium aurelia" Amer. Jour, of Physiol., vol. xxix., 1911. 



3 Calkins points out that the same experiment is performed by mosquitoes 

 and other insects on certain parasitic Protozoa, as when a parasite is with- 

 drawn from the hot environment of the Mammalian blood into the compara- 

 tively cold region of the mosquito's alimentary tract. ("The Protozoan 

 Life-Cycle," Biol. Bull, vol. xi., 1906.) 



4 Calkins, loc. cit. 



6 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 



