Size in Relation to Multiplication Rate 95 



metazoa such a requirement would never be connected 

 with regeneration; because it is there easy to observe 

 other processes which are more to the point than the 

 ability to have progeny. Among protozoa, in fact, 

 types of reorganization are well known to occur quite 

 without conferring the ability to survive indefinitely 

 by means of multiplication. When the statement is 

 made that the nucleus is indispensable for regenera- 

 tion among protozoa, it means that only nucleated 

 fragments will reconstitute individuals which will 

 multiply. This is universally the case among species 

 possessing discrete nuclei. 



But some species allow the nuclei to be fragmented 

 too, and this is probably what occurs when species 

 without discrete nuclei are injured. In many such in- 

 stances, two or more individuals, each of which will 

 give rise to endless descendants, may be artificially 

 produced at any time from a single vegetative organ- 

 ism. 



The presence of some cytoplasm is generally be- 

 lieved to be necessary for regeneration. This problem 

 is an academic one, since it could always be argued that 

 some small amount of cytoplasm accompanied any pos- 

 sible nuclear fragment. 



Least size. A question of frequent interest has 

 been: how small may a fragment be and still grow and 

 reproduce? Average results for the smallest viable 

 pieces are listed in table 8. All those species which 

 show recovery in pieces constituting 1 to 7 percent of 

 the original size are those characterized by possessing 

 long, beaded or bacillary nuclei. In species with glob- 

 ular nuclei it is almost impossible to trim away more 

 than one-fifth of the body (as in Frontonia) without 

 injuring the nucleus; and globular nuclei seem to be 

 peculiarly poor at quickly forming new surfaces and 

 thus preserving their contents, in contrast to elongated 



