774 MORPHOGENESIS 



by Lund (1917). Slightly starved Bursaria frequently undergo exten- 

 sive reorganizational changes distinct from those which accompany divi- 

 sion, conjugation, and so forth. According to Lund, this distinctive type 

 of physiological regeneration has no apparent extrinsic cause. That such 

 is the case seems improbable. Hetherington (1932) cultivated Stentor 

 coendeus under more carefully controlled conditions and was unable to 

 find cases of spontaneous regeneration except when the environment 

 was unfavorable. Starvation, wide variations in oxygen tension and or- 

 ganic content of the medium are apt to initiate these reorganizational 

 changes. Hetherington maintains that physiological regeneration occurs 

 very infrequently, if at all, and that it has never been demonstrated in 

 a known medium. Both Lund and Hetherington are inclined to employ 

 the term physiological regeneration in a special sense, not including the 

 cyclical changes already mentioned. 



Periodic reorganizations during periods of partial or complete starva- 

 tion in Stylonychia myt'ilis were recently reported by Dembowska ( 1938) . 

 In conductivity water Stylonychia lives for fourteen to nineteen days. 

 During this time the organisms undergo repeated processes of complete 

 body reorganization by renewing the entire ciliature and by nuclear re- 

 organization. The reorganizations are increasingly frequent up to a cer- 

 tain point and are unaccompanied by manifestations of division. 



Some of the Factors in Regeneration 

 1. external environment 



Calkins (1911b) and especially Peebles (1912) noted a definite cor- 

 relation between the regenerative behavior of Paramecium caudatum and 

 the periods of depression to which their cultures were subject. During 

 such periods pieces cut from organisms, which under more favorable 

 conditions exhibited a greater capacity to regenerate, were unable to 

 regenerate or to divide. These authors concluded that when paramecia 

 are starved or are undergoing periods of depression from other causes, 

 the division rate is greatly diminished and the reparative activities greatly 

 reduced or altogether lost. 



Diminutive but perfectly formed regenerates were obtained when 

 Sokolofi^ (1923) cut slightly starved Bursaria into several parts. These 

 regenerates were proportionate in size to the pieces from which they 

 were derived. In other words, within reasonable limits, inanition in- 



