VITALITY 479 



In other groups than the ciliates, exceptions to the type of life 

 history shown by Uroleptus are true of the few cases known. In 

 the animal flagellates for example there is no case of indubitable 

 proof of fertilization in the entire group. On the other hand there 

 have been no successful attempts to cultivate such flagellates by 

 the isolation culture method so that we are entirely uninformed 

 as to the relative vitality in a life cycle. It is possible that processes 

 analogous to endomixis in ciliates take place during encystment 

 stages but as to this we are also ignorant. With these exceptional 

 cases therefore we must wait for further information. With plant 

 flagellates, except for the Phytomonadida, the situation is the same ; 

 no complete life cycle has yet appeared and phenomena of encyst- 

 ment have not been studied in detail. In Phytomonadida fertiliza- 

 tion processes, often accompanied by sexual differentiation are 

 universal, and some excellent work by Hartmann (1921) with 

 isolation cultures has been carried out. Eudorina elegans was 

 followed for upward of five years on artificial culture media and 

 under constant artificial light for part of the time. Eudorina is a 

 colonial form of 32 cells embedded in jelly and upon reproduction 

 each of the 32 cells forms a similar 32-cell colony. Each period of 

 reproduction therefore represents 5 divisions of a cell, and in one 

 culture 1500 such cell generations were obtained. Knop's solution, 

 made with distilled water was constantly used and the cultures 

 were maintained free from foreign organisms. Some of Hartmann's 

 earlier series showed evident depression, decline of vitality and 

 death but this result was attributed to faulty conditions. 



Hartmann's records are based upon the length of the inter- 

 divisional period instead of upon the number of divisions per unit 

 period of time as in pure line work with ciliates. Thus the average 

 interval between generations for the first 10 generations was seven 

 and eighth-tenth days, for the fifth set of 10 generations the interval 

 increased to an average of seventeen and nine-tenth days, etc. 

 His results show clearly the effect of perfected environmental 

 conditions (last 90 generations) and give no indication of periods 

 of depression or of waning vitality. So far as this one series is 

 decisive therefore, there is justification for Hartmann's conclusion 

 that in Eudorina division apparently may continue indefinitely 

 in the absence of conjugation or fertilization and without waning 

 vitality. 



Exceptional cases are increased through Belaf's observations on 

 Actinophrys sol a heliozoon (1924). A single line of his main 

 culture was followed through 1244 generations by division during 

 two years and eight months. Fertilizations were obtained from 

 time to time in mass cultures but these were prevented in the 

 isolation cultures, the latter showing no indication of reduced 

 vitality with continued life (Fig. 201). Belaf also concludes that, 



