MORE COMPLICATED LIFE CYCLES HI 



A. The Period of Youth. — As with the fertilized egg of a metazoon, 

 this first period of vitahty of the copula or fertilized cell of a protozoon 

 is characterized by the distinct excess of constructive over destructive 

 metabolism, which indicates a high potential of vitality and great 

 powers of cell reproduction, which may take the form of division, 

 budding, or spore formation according to the difficulties successfully 

 overcome by the type in the struggle for existence. These young 

 forms show a well-marked conformity to type, and this feature, occur- 

 ring when the greatest numbers of representatives of the species are 

 in evidence, undoubtedly has given a false impression of the stability 

 of form of the protozoan species. The protoplasm, as a rule, is trans- 

 parent and without reserve matters, metaplasm products, and the like, 

 and the nucleus is often without the characteristic structures of the 

 later forms. 



It is along physiological lines that the young forms are most promi- 

 nently marked. This is the period, for example, of the greatest resist- 

 ance to adverse conditions in the surrounding medium, and in patho- 

 genic 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 infection, the failures being due, presumably, to 

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

 withstand the natural immunity of a new host. The matter of malig- 

 nancy is so intimately connected with restored vitality that in yellow 

 fever, for example, it is almost sufficient to indicate that fertilization 

 processes and renewal of vitality must have taken place in the body of 

 the intermediate mosquito host. 



At this period, also, is the greatest power of self-preservation in other 

 ways than by resistance of a chemical nature; thus, the firm protective 

 cysts are formed at this period within which the fertilized cell may 

 resist heat, cold, and drought, as in many of the free forms of protozoa 

 when the organisms live thus through the winter, or in parasitic forms 

 like the sporozoa, when the organisms are protected in the interval of 

 chantfinf; hosts. 



The difficulties in determining which are young and which older 

 cells of a life cycle are great, and much must be left for inference. It 

 may be accomplished, however, in one of several ways: (1) By culture 

 experiments for which cells are isolated immediately after conjugation, 

 a method that may be easily employed for the larger free protozoa. 

 (2) By inoculation of uninfected hosts with the spores of the form to 

 be studied, a method which may be employed with sporozoa or with 

 encysted amebae. (3) By natural inoculation through the opera- 

 tion of intermediate hosts, such as insects, ticks, or leeches. Few 

 observations, however, have been made upon the young forms, prob- 

 ably because the morphological characteristics of the mature cells are 

 much more apparent tiian those of the young. 



