530 GROWTH 



cyclic phenomena. Further study suggested that the seasonal cycle was 

 associated with sunlight (Richards, 1929). The pigmented Blepharisma 

 followed, more closely than the others, the seasonal variation of radiant 

 energy. A recent graphic method of Spurr (1937) could be used to 

 advantage in the analysis of seasonal cycles in the division rate of Pro- 

 tozoa. Properly controlled studies should be made to determine just how 

 much effect light has, over a considerable period of time, on the growth 

 of Protozoa. Such seasonal effect appears reasonable, as it is known that 

 the reproductive cycles of some birds and other animals are initiated 

 by the increased amount of light during the early part of the year. 



Conjugation restricts variation, which aids in survival during ad- 

 verse conditions, according to Pearl (1907). Endomixis occasioned 

 large variations in size, which Erdmann (1920) believed aided in sur- 

 vival. She advised that attempts at selection be made during or immedi- 

 ately after endomixis. Changes that aid in survival of a species through 

 an unfavorable period are important in population studies; they might 

 even effect the growth of the individual, and they deserve further in- 

 vestigation by protozoologists. Selection of rapidly dividing Amoeba 

 proteus by Halsey (1936) did not produce a permanent race of rapidly 

 dividing individuals. Burnside (1929) failed to change the size of 

 Stentor coeruleus by fragmenting animals with large and small amounts 

 of nuclear material. When the animals regenerated, the regulatory pro- 

 cesses produced the same sized biotype. 



Variations in the cell, at the time of division or during periods of 

 intercellular reorganization, may aid in the adaptation of the cell to a 

 new environment and may account for the success or failure of investi- 

 gators in acclimating an organism to life in a synthetic liquid culture 

 medium. Hegerty (1939) has shown that young Streptococcus Idct'is, 

 at the end of the lag period and just before the period of logarithmic 

 growth, can produce new enzymes which permit the use of a new sub- 

 strate, to which the bacteria could not adapt themselves at any other 

 period of their life cycle. Do comparable changes occur in Protozoa? 

 If so, pure culture methods would be facilitated. 



Thus the nature of the life cycles, as demonstrated by the earlier in- 

 vestigators (M. Robertson, 1929), may now be studied effectively by 

 physiological methods, as well as by post-mortem cytology. The de- 

 tailed discussion of reproduction must be left for consideration in the 



