778 MORPHOGENESIS 



regardless of the time of cutting during the division cycle. Calkins presented 

 exceptions to this corollary: fragments from cells cut immediately after division 

 which contained both types of nuclei did not regenerate. A more complete 

 history of fragments than he presented would, however, be necessary to 

 ascertain whether their failure was due to a decrease in the "power of regenera- 

 tion" or merely to unsuccessful recovery from the operation. These cases seem 

 to indicate that just after division animals may fail to regenerate even though 

 they contain the full nuclear complement; but such failures were certainly 

 the exception and not the rule (four cases out of twenty-two), so that demon- 

 stration of a fundamental decrease in the "power of regeneration" during 

 this period, exclusive of the subtraction of one of the nuclei, was not in- 

 tended [p. 199}. 



From data including only fifty operations made during and after 

 division in one race of Varamecium caudcitum, of which twenty-four suc- 

 cessfully regenerated and subsequently divided, Tartar concluded that 

 the division cycle in this species does not influence regeneration, even 

 when both nuclei are present. 



Evidence of progressive physiological differentiation between divi- 

 sional periods, other than the increasing ability to regenerate, is the 

 determination of the division plane. Lewin (1910), Calkins (1911b), 

 and Peebles (1912) have repeatedly demonstrated its occurrence in P. 

 caudattwT, other amply described cases are those of IJr onychia (Calkins, 

 1911a; D. B. Young, 1922; Dembowska, 1926) and Oxytricha (M. E. 

 Reynolds, 1932). All of these species may be transected above or below 

 the mid-region sometime prior to cell division, without injury to this 

 definitive region. As already described for Ur onychia, the larger piece 

 divides unequally through what was originally the mid-region of the 

 intact cell. Peebles identified the division plane as early as 2.5 hours 

 after fission. She thought that several division planes develop in vegeta- 

 tive cells when fission has been delayed for a time. This would account 

 for the fact that several divisions follow in rapid succession when one of 

 the extremities is cut away from such an individual. 



Conjugation. — Four giant races of Paramecium caudatum were re- 

 ported by Calkins (1911b) as having different regeneration potentials. 

 The incidence of regeneration in three of the races, those having very 

 restricted regenerative powers, was found to be greater in ex-con jugants, 

 or in cells that were operated during conjugation, than in the ordinary 

 vegetative individuals. 



The merotomy studies on Uroleptus mobilis (Calkins, 1921) proved 



