No. 593] 



THE PROTOZOAN LIFE CYCLE 



205 



addition to these casual encystments there is another form* 

 of encystment which involves more deeply-lying activities 

 of the protoplasm. In Didinium mis ut urn 1 have found 

 that encystment occurs at periodic intervals which cannot 

 in any way he connected with adverse conditions of the 

 environment or with feeding-, hut must be interpreted as a 

 normal phenomenon due to internal conditions of the 

 organisms. Encystment at such times persists for from 

 5 to 8 days and during this period no amount of coaxing 

 will bring the organisms out. 1 hiring such encystment the 

 macronucleus fragments into hundreds of small chromatin 

 particles which are ultimately absorbed in the cytoplasm ; 

 the micronuclei divide, and products of their division give 

 rise to a new macronucleus and new micronuclei. When 

 the process is completed and the organisms emerge from 

 their cysts they possess from five to seven times the vital- 

 ity, as measured by the division rate, of the same race 

 prior to encystment. Fermor was the first in 1913 to de- 

 scribe similar happenings during the encystment of Stylo- 

 nychia; in this case, dissolution of the old macronucleus 

 and absorption of the fragments, fusion of the two micro- 

 nuclei and formation of new macronuclei and micronuclei 

 from the fusion nucleus, were described. 



It is well known that Paramecium does not encyst. 

 Nevertheless Woodruff and Erdmann (1914) have shown 

 that phenomena similar to those occurring during encyst- 

 ment in Stylomjchia and Didinium, and which they refer 



intervals \ about once a month ) in the ra>.« of hu;i lt „ < ium 

 aurelia. Here also the old macronucleus fragments and 

 the fragments are absorbed in the cytoplasm, while a new 

 macronucleus and micronuclei are formed from the divi- 



