792 MORPHOGENESIS 



qualitatively homogeneous. Full regeneration is frequently possible when 

 only a small piece of the macronucleus is present. The rate of regenera- 

 tion in Bursaria is independent of the size of the macronuclear fragment 

 included. The two halves of an individual, one containing a small and 

 the other a very large portion of the macronucleus, usually regenerate 

 in about the same length of time (Lund, 1917). 



It is well established that micronuclei are capable of regenerating 

 other micronuclei. A unimicronucleate section from a multimicronucleate 

 species eventually regains the characteristic number, provided other con- 

 ditions favor regeneration. There has been no confirmation of Lewin's 

 (1912) experiments on Stylonychia in which, after merotomy, the micro- 

 nuclei increased in number beyond that characteristic of the normal races. 

 The effect of merotomy upon micronuclear number in Pleurotricha was 

 investigated by Hewitt (1914), who concluded that merotomy does not 

 permanently alter the numerical relationship between micronuclei and 

 macronuclei. If at least a representative of each is present in the frag- 

 ment, the normal nuclear complement occurs in the immediate regener- 

 ate and in its descendants. 



The question arises as to whether or not the inability of an experi- 

 mentally amicronucleated fragment to regenerate physiologically is like- 

 wise characteristic of the fragments derived from viable amicronucleate 

 races. The contribution of M. E. Reynolds (1932) definitely answers 

 the question: "The regeneration process in the fragments from the 

 amicronucleate Oxytricha is similar to that which occurs in pieces from 

 micronucleate Oxytricha which contain both types of nuclei" (p. 357). 

 The absence of the micronucleus as a structural entity does not alter the 

 course of regeneration. As in the normally constituted races, regeneration 

 is accompanied by a protoplasmic reorganization involving the dedif- 

 ferentiation of old and the redifferentiation of new ciliary fields from 

 migrating Anlagen. The extent and time of regeneration, as well as the 

 total number of regenerates obtained from the peculiar race of O. jail ax, 

 compare favorably with the normal strains. In view of the fact that ex- 

 perimentally amicronucleate fragments do not regenerate in the com- 

 plete sense, we are obliged to assume that whatever the constitution of 

 the usual micronucleus, it is somehow represented in the amicronucleate 

 races. Following Woodruff's (1921) interpretation, Reynolds regarded 



