72 AN INTRODUCTION TO ZOOLOGY 



of vitality, rapid cell multiplication and growth, and active 

 functions ; at maturity, the cell division becomes less rapid, 

 growth ends, and the organism becomes sexually mature; in old 

 age, cells break down, functions become imperfect, and degenera- 

 tion sets in, ending in natural death. These three stages are found 

 in the life cycle of Paramecium, provided we accept the above 

 definition of the protozoan individual; youth is characterized 

 by rapidly dividing cells; maturity by the attainment of full 

 size and conjugation; and old age by degeneration and natural 

 death if conjugation is prevented. 



In the stage of youth Paramecia do not conjugate even if many 

 are confined in a limited space. The animals divide very rapidly 

 at this time. They are usually almost transparent and free 

 from reserve material of all kinds, and are able to resist adverse 

 conditions, showing a high grade of vitality; this is due to the 

 excess of constructive over destructive metabolism. There is 

 no definite limit to youth. Maturity comes on imperceptibly, 

 and mature animals can be recognized only when in some phase 

 of sexual activity. 



In a culture under continual observation a decline in the rate 

 of reproduction indicates the approach of maturity. The proto- 

 plasm at this stage undergoes a change both physically and 

 chemically; the surface layer becomes sticky, so that when two 

 cells meet they fuse, and conjugation results. This frequently 

 occurs in a large number of animals in a single culture at the same 

 time and a so-called " epidemic " of conjugation may then be 

 observed. The conjugants are smaller than the other specimens, 

 being only .21 mm. long, while the usual length is about .3 mm. 

 The immediate result of conjugation is apparently the rejuvena- 

 tion of the conjugants. 



If the Paramecia are kept in a constant medium, e.g. " hay in- 

 fusion," they undergo a period of physiological depression about 

 every three months, as shown by the decrease in their rate of 

 division. These periods of depression are due to unknown meta- 

 bolic conditions, which lessen the rapidity of division, but do not 



