6 THE PHYSIOLOGY OF REPRODUCTION 



the breeding season, giving illustrations, taken from various 

 groups of Vertebrates and Invertebrates, of its seasonal re- 

 currence, and the manner in which this varies under altered 

 conditions of life. 



PROTOZOA 



Among the Protozoa the organisms pass through successive 

 phases of vitality, which are comparable to the different age- 

 periods of the Metazoa. In such simple forms of life, fission or 

 division into two parts is the usual method of reproduction, 1 

 and the frequency of its occurrence appears to depend more 

 upon the phase which has 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 con- 

 jugating individuals ; and finally, in forms which do not con- 

 jugate, 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 Infusiorian, Stylonychia pustulata, if 

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

 a temperature of from 5 to 10 C., and once in twelve hours 

 in a temperature of from 10 to 15 C. 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 Para- 

 moBcium may be renewed without the occurrence of conjuga- 

 tion, that is to say, fission can be made to continue and 

 senescence can be avoided, by introducing a change in the 



1 In this process no material is lost, and two simple nucleated organisms 

 result. During the period of maturity referred to in the text, multiplication 

 is often preceded by union (either temporary or complete) of two individuals, 

 and this process is called conjugation (see p. 211, Chap. VI.). 



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



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.) 



