REPRODUCTION 



179 



an increased internal pressure due to the imbibition of water, re- 

 sults in the rupture of the cyst wall which had lost its rigidity and 

 resistance (Fig. 75). Apertures in the cyst wall of Pelomyxa illi- 

 noisensis are apparently produced by pseudopodial pressure (Kudo, 

 1951). Seeing a similar aperture formation in the cyst of Entamoeba 

 histolytica, Dobell (1928) "imagined that the amoeba secretes a fer- 

 ment which dissolves the cyst wall." 



Fig. 75. Excystment in Didinium nasutum, as seen in a single indi- 

 vidual, X250 (Beers), a, resting cyst; b, appearance of "excystment" 

 vacuole; c, rupture of the cyst membrane, the vacuole is becoming en- 

 larged; d, e, emergence of the cyst content, the vacuole increasing in 

 size; f, the empty outer cyst membrane; g, the free organism with the 

 inner membrane; h, organism after discharge of vacuole; i, j, later stages 

 of emergence of the ciliate. 



Although encystment seems to be an essential phase in the life 

 cycle of Protozoa in general, there are certain Protozoa including 

 such common and widely distributed forms as the species of Para- 

 mecium in which this phenomenon has not been definitely observed 

 (p. 744). In some Sporozoa, encystment is followed by production 

 of large numbers of spores, while in others there is no encystment. 

 Here at the end of active multiplication of trophozoite, sexual re- 



