REPRODUCTION AND THE LIFE CYCLE 187 



It was shown in the hist chapter tliat paraniecium could be restimu- 

 lated, even (hn-ing a well-advanced perioil of depression, by means of 

 salts of different kinds. Such stimulation, preventing natural physio- 

 logical death of the organism, is analogous to the artificial partheno- 

 genesis by use of salts in the case of eggs of sea urchins and star fish. 

 The researches of IMorgan, Loeb, AVilson, Delage, and others have 

 shown that fertilization is not necessary for development of the egg 

 of such forms. So in the case of paraniecium and other ciliates in 

 which the life history has been followed out, the use of a new medium 

 with some appropriate salt effected the same reaction as salts do in 

 artificial parthenogenesis. The observations upon the lower organ- 

 isms went a step farther, however, by showing that in paraniecium 

 such stimulation could not be continued indefinitely, a time coming 

 when the stimulants failed to produce the effect previously obtained. 



So it may be with the blood parasites; some of them, like the malaria 

 organisms, niav be artificiallv stimulated bv some minute change in 

 the constitution of the blood, and so bring about a relapse (see Calkins, 

 190()). Parthenogenesis, effecting the same end, has been described 

 by Schaudinn in the case of Plasmodium vivax, the cause of tertian 

 fever in man, and in the case of Trypanosoma noctuce, a blood parasite 

 of the little owl (see p. 163). 



^"ariations in the endogenous cycle of parasites thus have to do 

 mainly with the methods of asexual increase. The more primitive 

 forms of parasites, i. e., those which have most recently adopted the 

 parasitic mode of life, still reproduce as do the free-living or non- 

 parasitic types. In other forms simple division is replaced by more or 

 less prolific methods of brood formation, in response, proba]>ly, to the 

 needs of the race, and methods which culminate in fully developed 

 schizogony, usually s(>rving as a means of auto-infection. 



2. Variations in the Exogenous Cycle. — The exogenous cycle 



begins with the fertilization of the cell and formation of the external 



spore coverings within which the young organisms are protected from 



adverse conditions. There is reason to believe that such protective 



structures and adaptations of the exogenous cvcle are distinctlv 

 «... r^ , V 



characteristic of the period of youth in the life history, and due in large 

 part to the high potential of vitality which distinguishes the fertilized 

 cell from all others. The reproductive processes involved are certainly 

 more romplicatcd than those of the endogenous cycle, and ai'e more 

 definitely correlated with the j)erj)etuation of the race. 



In the simplest cases the fertilized cell forms a chitinous spore 

 covering which, with desiccation, may become hard and resistant, 

 while no internal nuclear or cytoplasmic processes take place. When 

 taken again into a new host, where conditions are favorable for the 

 dissolution of the cyst, a single, young, iind uninucleate" j)arasite 

 emerges. Such is the condition in many of the parasitic llagellates 



