142 



PROTOZOOLOGY 



Fig. 66. Encystment of Lophomonas hlattarum, X1150 (Kudo). 



group of individuals of one species, usually of Sarcodina, which 

 become attached to one another by means of pseudopodia in an 

 irregular form. 



Asexual reproduction 



The Protozoa nourish themselves by certain methods, grow 

 and multiply by the methods described in the preceding pages. 

 This phase of the life-cycle of a protozoan is the vegetative stage 

 or the trophozoite. The trophozoite repeats its asexual reproduc- 

 tion process under favorable circumstances. Generally speaking, 

 the Sporozoa increase to a much greater number by schizogony 

 and the trophozoites are called schizonts. 



Under certain conditions, the trophozoite undergoes encyst- 

 ment (Fig. 66). Prior to encystment, the trophozoites cease to 

 ingest, and extrude remains of, food particles, resulting in some- 

 what smaller forms which are usually rounded and inactive. This 

 is often called the precystic stage. The organism presently 

 secretes substances which become solidified into the cyst wall 

 and thus the cyst is formed. In this condition, the protozoan ap- 

 parently is able to maintain its vitality for a certain length of 

 time under unfavorable conditions. The causes of encystment are 

 still the matter which many investigators are attempting to com- 

 prehend. It appears certain at least in some cases that the encyst- 

 ment is brought about by changes in temperature, desiccation, 

 and chemical composition, amount of food material, accumulation 

 of catabolic wastes, etc., in the medium in which the organisms 



