MORPHOGENESIS 777 



the progressive development of regenerative power from one division 

 to the next, Calkins's experiences with Uronychia have been repeated and 

 reaffirmed by D. B. Young (1922) and Dembowska (1926). Similar 

 results were also reported by M. E. Reynolds (1932) for an amicro- 

 nucleate race of Oxytrkha jallax. 



To the contrary, in Spathidhim there appears to be no correlation be- 

 tween the degree of regeneration and the division cycle. Before, during, 

 and after fission, small pieces excised from either extremity disintegrate 

 immediately or regenerate without dividing. Large nucleated fragments 

 nearly always regenerate, irrespective of the stage at which they are 

 taken (Moore, 1924). 



Peebles (1912) cut Paramecium caudatum at two different division 

 stages without discovering any marked differences in the regenerative 

 ability. Both halves of those which were transected in the division plane, 

 at a time when the macronucleus was elongated and the body slightly 

 constricted, usually survived to produce normal descendants. Traumatic 

 effects were reduced in operations made during the later stages. Peebles 

 also made note of the fact that while the power of regeneration is present 

 in cells obtained from two to five hours after separation, approximately 

 90 percent of the operated individuals died as a result of injury. This 

 was attributed to lowered viscosity in the cytoplasm of growing para- 

 mecia, such that an injury to the ectosarc allows the endoplasm to escape. 

 The surviving organisms regenerated as readily as those cut in the late 

 interphase. 



Taking exception to Calkins's statement that the "power of regenera- 

 tion" varies in different stages of the division cycle, Tartar (1939) 

 restates the problem in terms of the observed data, without reference to 

 the "power" of regeneration. Calkins's experiments 



reveal that specimens of Uronychia transfuga are able to regenerate mor- 

 phologically without the presence of the micronucleus when the transection 

 removes this structure before or during division, and not when it is removed 

 after division. The fate of the amicronucleate fragments was apparently not 

 followed long enough to determine to what extent subsequent division is 

 possible in the absence of the micronucleus, but it is probable from the work 

 of others (e.g. Moore, 1924) that division would not have taken place 

 without the micronucleus. It is for this reason that my restatement of Calkins' 

 results restricts the restoration to morphological regeneration. A corollary of 

 the statement is that when a fragment contains both nuclei, it regenerates 



