AN AMICRONUCLEATE OXYTRICHA. II 151 



Further, the conditions necessary for the production of a race 

 which at some time in its life-history will possess a dorsal spine 

 (Jennings, '08) are, in a manner, realized in each series of twin 

 animals. A twin frequently divided to give a typical twin 

 from the posterior end and from the anterior end a twin widely 

 separated anteriorly. This twin invariably pulled apart pro- 

 ducing two single animals with pronounced dorsal spines (fig. 

 21), It is true that these spines were always soon resorbed, but 

 the production of such individuals occurred regularly throughout 

 the life-history of each twin series, so that what might be con- 

 sidered the establishment of a 'youthful' characteristic was 

 transmitted from generation to generation, though not visible 

 in the adult cell. 



In part I of this study the conclusion was reached that, given 

 favorable environmental conditions, the race would live indefi- 

 nitely not only without conjugation, autogamy, or endomixis, 

 but apparently without the ability to undergo these phenomena. 

 The same conclusion seems equally applicable to the various 

 races of double animals or twins. It is, of course, impossible 

 to continue to breed the twins indefinitely in small mass cul- 

 tures, since twins are invariably eliminated when bred in com- 

 petition with single animals. It should be noted, however, that 

 in the case of the longest-lived twin culture (AT) the twin forms 

 outlived the culture of single anijnals (A) from stock of which 

 they were derived. 



Since the formation of double animals which reproduce con- 

 sistently by fission to produce similar progeny for many genera- 

 tions under suitable cultural conditions is nowhere recorded in 

 the annals of the Ciliates, it may be suggested, perhaps, that they 

 are abnormal and, further, that the race on which the experi- 

 mental study has been made is itself abnormal, inasmuch as all 

 the individuals of the race are amicronucleate. If, however, the 

 term abnormal is used to connote a pathological condition, its 

 application to the phenomena described is highly misleading. 

 The phenomena which have been described, i.e., pairing, cannibal- 

 ism, and formation of double animals, are entirely 'normal' so 

 far as this particular race is concerned, since such phenomena 



