REPRODUCTION 



175 



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 reproduction process 

 under favorable circumstances. Generally speaking, the Sporozoa 

 ncrease to a much greater number by multiple division or schizog- 

 ony and the trophozoites are called schizonts. 



Under certain conditions, the trophozoite undergoes encystment 

 (Fig. 72). Prior to encystment, the trophozoites cease to ingest, and 

 extrude remains of, food particles, resulting in somewhat smaller 

 forms which are usually rounded and less active. This phase is some- 



Fig. 72. Encystment of Lophomonas blattarum, X1150 (Kudo). 



times called the precystic stage. The whole organism becomes de- 

 differentiated; namely, various cell organs such as cilia, cirri, 

 flagella, axostyle, peristome, etc., become usually absorbed. Finally 

 the organism secretes substances which become solidified into a re- 

 sistant wall, and thus the cyst is formed. In this condition, the 

 protozoan is apparently able to maintain its vitality for a certain 

 length of time under unfavorable conditions. 



Protozoa appear to encyst under various conditions. Low tem- 

 perature (Schmahl, 1926), evaporation (Belaf, 1921; Bodine, 1923; 

 Garnjobst, 1928), change in pH (Koffman, 1924; Darby, 1929), low 

 or high oxygen content (Brand, 1923; Rosenberg, 1938), accumula- 

 tion of metabolic products (Belaf, 1921; Mast and Ibara, 1923; 

 Beers, 1926) or of associated bacteria (Mouton, 1902; Belaf, 1921) 

 and over-population (Barker and Taylor, 1931) in the water in which 

 Protozoa live, have been reported to bring about encystment. While 



