506 BIOLOGY OF THE PROTOZOA ' 



state characteristic of the early inter-divisional period. If the 

 reorganizations effected by these di\isional activities are always 

 the same generation after generation, then, on the hypothesis, 

 there is no a priori reason why under appropriate environmental 

 conditions, metabolic activities, or vitality, should not continue 

 indefinitely (See Child, Hartmann, Belaf, Jollos, etc.). Such is 

 the explanation that I would give of continued life without fertiliza- 

 tion of animal flagellates, aided here possibly, by changes which 

 may take place during the periods of encystment. On the same 

 })asis we find an explanation of the long-continued isolation cultures 

 without fertilization of organisms which, under usual conditions, 

 undergo fertilization. Some types of organization are evidently 

 able under appropriate conditions of the environment to return to 

 the same labile organization after each division. Such types would 

 thus have a prolonged asexual cycle, possibly, as Enriques asserts, 

 as long as the observer cares to continue the culture. Here Fyudorina 

 elegans (Hartmann) and Actinophrys sol (Belaf) are conspicuous 

 exami)les to which should be added the exceptionally long-lived 

 races of Infusoria in the hands of Enriques, and of Woodruff and 

 his pupils. Here also we may add the amicronucleate race of 

 Didinium nasutum which Patten (1921) followed through 652 

 generations in two hundred and thirty-eight days, and one series 

 of Uroleptus mohiUs which lived through a period of five hundred 

 and ninety-eight days, divided 597 times and had a relative vitality 

 of 110.4 per cent, although such cases may be interpreted as due to 

 peculiar combinations through amphimixis. 



If, however, reorganization as effected by division does not leave 

 the i^rotoplasm in its original labile condition, then inter-divisional 

 activity of the progeny starts with a different organization than did 

 the previous generation and this, continued generation after genera- 

 tion produces an accumulati\'e effect. This is manifested by phy- 

 siological activities and by structural modifications not shown before. 

 The decline in the division-rate for example, may indicate that the 

 living substances are becoming relatively stabile and more and 

 more irreversible in phase as was the case with one race of Para- 

 vterium ravdafum in Mdiich the individuals became homogeneous 

 and black in appearance with complete loss of the usual vesicular 

 character (Calkins, 1904, Eig. 212). This particular condition was 

 relieved by the use of electrolytes added to the usual medium 

 (K2HPO4, KCl, etc.). In extreme old age in ciliates there is appar- 

 ently a cessation of the intricate activities involved in cell division. 

 Evidence of this is the tendency to form monsters and the tendency 

 of parts to undergo degeneration, nuclei, motile organs, kinetic 

 elements, etc, in particular. 



Between the extremes of youth on the one hand and old age on the 

 other is a condition of cumulative differentiation termed sexual 



