Reproduction and Life-Cycles 77 



in excystment. The resulting increase in volume forcibly ruptures rigid 

 membranes. In ciliates, the absorbed water may accumulate in a large 

 excystment-vacuole (Fig. 2. 16, A), apparently identical with the con- 

 tractile vacuole of Euplotes taylori (90) and Didinium nasutum (12), or 

 in a number of vacuoles as in Tillina magna (11). The second mechanism 

 involves the secretion of enzymes which digest the endocyst and perhaps 

 other flexible membranes. This enzymatic action, first described in Col- 

 poda ciicuUus (100), probably occurs also in Tillina and Didinium. 

 The first signs of excystment in Didinium nasutum (Fig. 2. 16) are the 



Fig. 2. 16. Exc)stment in Didinmm nasutum; x275 (after Beers). A. 

 Appearance of excystment-vacuole. B. Ectocyst and mesocyst almost rup- 

 tured. C. Endocyst protruding from ruptured outer membranes. D. Organ- 

 ism after discharge of excystment-vacuole. E, F. Active ciliate in endocyst, 

 which increases in diameter. G. Escape of ciliate. Key: ec, ectocyst; en, 

 endocyst; in, mesocvst; v, excystment-vacuole. 



beginning of cyclosis and the appearance of a small posterior vacuole. 

 When the vacuole grows to about half the volume of the body, a bulge 

 appears at the opposite pole of the cyst. A little later, the mesocyst and 

 ectocyst are ruptured and the organism slips out, still within the endo- 

 cyst. The ciliate soon becomes very active within the endocyst, which 

 gradually increases in diameter. The membrane becomes thinner and 

 thinner, and finally seems to dissolve in the medium. Excystment is com- 

 pleted within four hours. At emergence, the meridionally arranged cilia 

 extend from the anterior ciliary girdle about halfway to the posterior 

 end of the body. Later on, the posterior cilia of the longitudinal rows 



