132 PHYSIOLOGY OF PROTOZOA 



place. Great care was taken to keep the culture medium favourable, 

 and it was found that the ciliates were just as vigorous at the end 

 of this period as was the original parent. It is thus evident that, 

 even in the case of an organism which under natural conditions conjugates 

 from time to time, the race may survive and still remain in vigorous 

 condition when this is prevented. Woodruf! (1921) showed that during 

 this period of repeated binary fission the process of renewal of the macro- 

 nucleus from the micronucleus, known as endomixis, took place at intervals 

 (see p. 54). In the case of the ciliate Spathidium spathula, Woodrui? 

 and Moore (1924) have demonstrated that reproduction can be continued 

 indefinitely without recourse to endomixis or conjugation when suitable 

 environmental conditions are supplied. 



From the work of Richard Hertwig and Maupas, who considered that 

 conjugation was essential to survival of the race, arose the theory of 

 rejuvenescence, which supposes that any race of ciliates dies out through 

 loss of vigour if conjugation does not take place. It has generally been 

 assumed that both these observers thought that the rejuvenating process 

 showed itself in an increase in the rate of multiplication. According to 

 Jennings (1920) this is a misrepresentation of their views, for it was 

 definitely stated that the rate of fission before and after conjugation was 

 not altered. Their view of the change which takes place in conjugation 

 is that the ciliates which would otherwise have died now continue to live, 

 and this continued existence itself is a sign of rejuvenescence. Calkins 

 (1919a), however, has definitely asserted that the failing energy and rate 

 of multiplication of the pre-conjugation period is abolished by conjugation, 

 and that in the post-conjugation period the rate of multiplication is 

 increased. Quite recently Woodruff and Spencer (1924), working with 

 Spathidium spathula, have clearly shoAvn that conjugation actually does 

 increase the rate of multiplication, and, furthermore, that on an average 

 cultures made from forms which have conjugated outlive those from forms 

 which have not, so that the chances of any particular line surviving are 

 increased. Careful experiments have not only shown that conjugation 

 is not necessary to continued existence, but appear to have demonstrated, 

 in many cases, that following it there is actual depression as regards rate 

 of division, likelihood of death, and in other respects. If conjugation does 

 not lead to some change of this kind, it is extremely difficult to account 

 for the process of syngamy at all. It appears to be unnecessary, yet it 

 takes place in nature. Minchin (1912) expressed the opinion that it 

 tends to level down individual variations and keeps the species true to 

 type. The true explanation may, however, be the reverse of this, as 

 Jennings (1920) has pointed out. 



It has been explained above that there occurs a reduction in the number 



