VITALITY 4S9 



occur only at the end phases of the Hfe cycle and are specific char- 

 acteristics of age. We find justification therefore, for purposes of 

 description at least, in presenting facts concerning differentiations 

 of youth, of maturity and of age, but we have no intention of set- 

 ting limits to these phases or of assuming that an ageing process is 

 inherent in Protozoa. 



A, Cyclical Differentiations Peculiar to Youth. — Intensity of 

 metabolic activities is one of the characteristic features of young 

 organisms but with Protozoa exact data are difficult to get except 

 from isolation cultures. In such cultures intensity is indicated 

 by the division-rate and the great majority of ciliates show a higher 

 division-rate in the early periods of vitality (see p. 558 and F'igs. 

 198 to 200). In Uroleptus inohilis this intensity lasts for approxi- 

 mately sixty days (Fig. 198) and in Spathidiuvi spathuJa for about 

 forty days (Fig. 200). The e\-idence is not consistent however if 

 all isolation cultures are considered and in exceptional cases of 

 Uroleptus and of Spathidivm there is no indication of this relative 

 intensity. Nor does Belaf give any evidence of it in his isolation 

 cultures of Actinophrys sol; nor does Hartmann (1921) for Eudorina 

 elegans. With parasitic forms exact data in this matter are wanting 

 and general impressions are of little value. 



Young organisms show the effects of abnormal conditions of the 

 environment more quickly and more intensely than do older ones. 

 Gregory (1925) for example has shown that salts and change of 

 medium are deleterious to very young forms of Uroleptus mobilis 

 while older forms are not affected. This is in line with Child's 

 results in connection with the action of potassium cyanide on many 

 kinds of organisms, those parts which ha^'e the highest metabohc 

 rate being first to succumb. 



The differentiations indicated above are physiological in nature 

 and are rather intangible. Other differentiations characteristic 

 of youth while also physiological are indicated by morphological 

 or structural modifications. Of these the most noteworthy are 

 the different types of cysts which are secreted by all kinds of 

 Protozoa. Encystment has been generally regarded as a means 

 of protection for the organism against adverse conditions of the 

 environment. This is probably more traditional than accurate, for 

 very few Protozoa are actually known to encyst when the exter- 

 nal conditions are unfavorable. IMast (1923) for example finds 

 that food and temperature have little effect in causing Didinium 

 nasutum to encyst but encystment takes place under the best 

 conditions. It is more probable that organisms which have hafl 

 the power to encyst persist under such conditions while the great 

 majority are killed. Cutler (1919) however gives evidence to show- 

 that skatol induces encystment in Endamceba dysenterioe. This 

 power to encyst appears to be a factor of young organisms induced 



