UNICELLULAR ORGANISMS 69 



even ignorant as to its exact purpose, although it apparently 

 appears to be a process necessary to reinvigorate the race and 

 prevent it from dying out under the ordinary conditions of 

 environment. The process is evidently closely associated with 

 sex reproduction in the higher animals and plants, which is to be 

 taken up in a later chapter. We may even speak of the youth 

 and maturity of a Paramecium; by the term youth meaning the 

 period of rapid cell division that follows conjugation, and by 

 maturity and old age, the period of slower cell division that 

 appears later in the life cycle of the animal. Possibly we may 

 say that the animal eventually dies of old age, by which we 

 would mean that unless conjugation occurs the process of simple 

 division is brought to an end by exhaustion. Whether old age, 

 and therefore conjugation, are necessary in the life history of 

 Paramecium is not yet settled. Experiments have seemed to 

 show that under proper conditions fission may go on almost 

 indefinitely, certainly up to 2500 cell divisions, without the 

 necessity of conjugation, or without seeming to produce any 

 impairment in the power of division. In the normal life of the 

 individual it appears that conjugation is required, however, by 

 some of the conditions of life. Paramecium, therefore, has a 

 definite life cycle, although we do not know its possible length 

 or the conditions which modify it. 



PLASMODIUM MALARIA 



As an example of a still more minute animal, we will study the 

 malarial organism, Plasmodium malarice, which lives in the hu- 

 man body. Human blood contains minute circular disks known 

 as red blood corpuscles (see page 192), within which the malarial 

 organisms may be found in persons who are suffering from 

 malaria, or chills and fever. The organism first appears as an 

 extremely minute body (Fig. 25 a), in shape somewhat like 

 the Amoeba, though much smaller. It increases in size as 

 shown by the successive figures a to e. After reaching a size 

 which nearly fills up the red blood corpuscles, it breaks up 



