THE PROTOZOAN LIFE CYCLE. 233 



tozoa dies out from protoplasmic senility no less surely than 

 does the body of the metazoon. We can distinguish, then, in 

 the life history of a protozoon three more or less clearly marked 

 stages, youth, adolescence, and old age, each with certain char- 

 acteristics but which we cannot sharply mark off one from an- 

 other any more than we can clearly limit the three stages of a 

 metazoon. 



As with the fertilized egg of a metazoon, the copula or ferti- 

 lized cell of a protozoon is endowed with a great power of cell- 

 reproduction and with a high potential of vitality, and this is the 

 main characteristic of the first period of the life cycle. This 

 reproduction may take the form of simple division, of budding, 

 or of spore formation, according to the difficulties that have been 

 successfully overcome by the species in its struggle for exist- 

 ence. The young forms show well marked conformity to type, 

 and this feature, occurring when the greatest numbers of repre- 

 sentatives of the species are in evidence, undoubtedly has given 

 a false impression of the stability of form of the protozoan species. 



This is the period also of the greatest resistance to adverse 

 conditions of the surrounding medium and in pathogenic forms 

 it is the period of greatest malignancy. It is a well-known fact 

 that, in many parasitic forms of protozoa, attempts to inoculate 

 from animal to animal are either failures altogether or result in a 

 weakened race of the organisms ; these failures are perhaps due 

 to the inability of the organisms in a more or less weakened 

 condition to withstand the natural immunity of the host, which 

 they are perfectly able to do with the full potential of vitality 

 with which they are endowed after conjugation. In some cases, 

 as for example in Trypanosoma, the natural vitality of the para- 

 site is so much greater than the natural resistance of the host 

 that such inoculation is possible and the transplanted organisms 

 continue to live. The matter of malignancy is so intimately con- 

 nected with this question of restored vitality that, in yellow fever 

 for example, it alone is almost sufficient to indicate that conjuga- 

 tion processes must take place in the body of Stcgomyia fasciata. 



This first period is then marked by a distinct excess of con- 

 structive over destructive metabolism and in the series of divisions 

 or repeated spore formation which follows fertilization there is a 



