238 GARY N. CALKINS. 



protozoa, as when some blood dwelling parasite is withdrawn 

 from the hot environment of the mammalian body to the colder 

 regions of a mosquito's digestive tract. If such experiments 

 become obligatory, and it is apparently so in many cases, then 

 certainly the most efficient prophylaxis is getting rid of the all- 

 important intermediate host, a preventive measure that has been 

 so signally successful in the Roman Champagna, in Cuba, in 

 Vera Cruz, and in New Orleans. 



The happenings within the body of such intermediate hosts 

 are by far the most important to the parasitic protozoa of all their 

 life processes, for the conjugation period with them, as with all 

 free-living protozoa, is the critical period of the life history, and 

 on it depends whether the race shall be given a new vigor and a 

 new lease of life, or shall pass on into the third period of the life 

 cycle characterized by old age or senescence and death. 



The third period in the life cycle of protozoa is characterized 

 by the peculiar cytolytic processes that accompany starvation, 

 by loss in size, by vacuolar degeneration in nucleus and cytoplasm, 

 and by final natural death. The symptoms may precede both 

 physiological and germinal death, and many of the so-called 

 involution forms frequently described in parasitic protozoa may 

 be individuals in this third stage of vitality. At this period, con- 

 jugation, as pointed out by Maupas, seems to be impossible, the 

 chance of rejuvenescence is cut off, and the race, now comparable 

 to the worn out somatic cells of a metazoon, becomes extinct. 



This series of changes from the fertilized cell to the ultimate 

 extinction by natural death is a consecutive series and forms a 

 clean-cut and well-defined life cycle or unit for all forms of pro- 

 tozoa. The vital processes are vegetative in nature and varying 

 phases may be largely accounted for by the conditions of metab- 

 olism. In metazoa we can make a clear distinction between 

 the history of the individual and the history of the race, and in 

 protozoa, with the life cycle as the unit, we can make the same 

 distinction. The 'ordinary phenomena of vegetative life of the 

 cell, metabolism in all its processes, have to do with digestion, 

 excretion, irritability, growth, and multiplication, and are functions 

 pertaining distinctly to the life cycle and may be considered in- 

 dependently of those which have to do with the continuity of the 



