476 BIOLOGY OF THE PROTOZOA 



old age indicated b;\' morphological degeneration with accumulating 

 physiological depression which ends in death. 



The many different series studied furnish ample opportunity for 

 the comparison of vitality in different series. In some there is a 

 greater intensity of vitality, /. c, the average division-rate is higher 

 throughout the cycle; in others the endurance factor is greater, 

 I. e., the individuals live for longer inter-divisional periods without 

 division and the cvcle is correspondingh' lengthened (see Chapter 

 XII). 



On the l)asis of such consistent experimental results one is tempted 

 to generalize and to hold that all Protozoa pass through a similar 

 life history. The temptation is increased by the confirmation of 

 the main results in connection with an entirely different ciliate, 

 Spathidium spathiila, in the hands of a no-le,ss competent observer 

 than Woodruff" (Woodruff' and Spencer, 1924). Spathidium is 

 carnivorous and feeds normally on Colpidium colpoda. Woodruff' 

 and Spencer's isolation cultures were carried on in a basic medium 

 of standardized beef extract to which a few indi\iduals of Coljyidium 

 were added. The individuals were transferred daily to fresh medium 

 and new food. Many complete series were followed from ex-con- 

 jugants, four lines to a series until the protoplasm died a natural 

 death. A typical example is illustrated in Fig. 200, representing 

 the division-rate averaged for five-day periods (solid line) and one 

 off'spring series. "The data presented show that in the great 

 majority of cases the cultures died out sooner or later after a some- 

 what gradual decline in the division-rate" (loc. cit. p. 178). Seventy- 

 nine series ran synchronously with their parent series for at least 

 fifteen days; some of these were then discarded but enough were 

 followed through to aff'ord a justifiable basis for conclusions. Here 

 then we have again a large number of series carried on in isolation 

 cultures, all derived from the same ancestral single ex-conjugant, 

 and dying out "after a somewhat gradual decline in division-rate." 



Woodruff', however (loc. cit.), does not grant that the decrease 

 in vitality is due to any intrinsic ageing tendency in the protoplasm 

 but believes that both in Uroleptus and in Spathidium the proper 

 milieu for continued life has not been provided in the culture methods 

 used, and implies that when a series dies in the absence of conjuga- 

 tion or of endomixis, it is ipso facto evidence of a faulty en^•ironment. 

 The matter is important for, if Woodruff"'s conclusion is correct it 

 brings us to an impasse in the subject luider discussion. He 

 supports his argument with the citation of cases on record in which 

 there is no evident diminution in the division-rate under the condi- 

 tions of culture, and in such cases he believes that natural environ- 

 mental conditions have been supplied. He obtained some cases 

 of greater longevity in a few series of Spathidium, and although the 

 methods and the culture medium sup])lied did not diff'er in any way 



