No. 593] THE PROTOZOAN LIFE CYCLE 261 



giant cirri at the posterior end (Fig. 1). Under labo- 

 ratory conditions it divides once a day approximately, or, 

 more exactly, once in twenty-six hours. The first indica- 

 tion of division is the precocious formation of the giant 

 cirri in a central region of the body which we have called 

 the "division zone." The experiments were undertaken 

 for the purpose of studying the relative power of regen- 

 eration of the single cell at different ages between divi- 

 sions, it having first been determined that the cell regen- 

 erates readily after being cut. Cells were cut with a scal- 

 pel at different periods subsequent to division; some 

 during the end stages of division ; some 15 minutes after 

 division ; some one hour after ; others 2, 4, 8, 12, 16 and 

 20 hours after, and some were cut just prior to the next 

 division period, i. e., 24 to 25 hours after division. In all 

 cases of record, the cells were so cut that one portion con- 

 tained the micronucleus and part of the macronucleus, the 

 other portion containing only a part of the macronucleus. 

 The former, or, as I shall call it, the nucleated portion, 

 invariably regenerated after some hours, forming a per- 

 fect cell, the latter, without a micronucleus which I shall 

 call the enucleated portion, behaved differently as regards 

 regeneration, according to the age of the cell when cut. 

 In all cases this portion lived from three to five days after 

 the operation. If the recently divided cell were cut at 

 any period up to 16 hours after division the result was the 

 same; no regeneration occurred, the fragment merely 

 rounded out, swimming about bv its adoral membranelles 

 (Fig. 2, 3). If the cells were cut when from 18 to 24 

 hours old, regeneration occurred not only in the nucleated 

 portion, but in the enucleated fragment as well, the per- 

 centage of regeneration increasing with the increased age 

 of the cells when cut, until at the age of 24-25 hours the 

 enucleated fragments regenerated perfectly in 100 per 

 cent, of cases (Fig. 4, 5, 6, 7). 



These results indicate a gradual chemical differentia- 

 tion of the protoplasm as a result, probably, of destruc- 

 tive and constructive metabolic processes. The giant 



