REPRODUCTION 171 



changed. Verworn considered that in these testaceans test-forming 

 activity of the nucleus is limited to the time of asexual reproduction 

 of the organisms. On the other hand several observers report in 

 Foraminifera the broken shell is completely regenerated at all times. 

 Verworn pointed out that this indicates that here the nucleus con- 

 trols the formation of shell at any time. In a radiolarian, Thalassi- 

 colla nucleata, the central capsule, if dissected out from the rest of 

 body, will regenerate into a complete organism (Schneider). A few 

 regeneration studies on Sporozoa have not given any results to be 

 considered here, because of the difficulties in finding suitable media 

 for cultivation in vitro. 



An enormous number of regeneration experiments have been con- 

 ducted on more than 50 ciliates by numerous investigators. Here 

 also the general conclusion is that the nucleus is necessary for re- 

 generation. In many cases, the macronucleus seems to be the only 

 essential nucleus for regeneration, as judged by the continued divi- 

 sion on record of several amicronucleate ciliates and by experiments 

 such as Schwartz's in which there was no regeneration in Stentor 

 coeruleus from which the whole macronucleus had been removed. 



A remarkably small part of a protozoan is known to be able to re- 

 generate completely if nuclear material is included. For example, 

 Sokoloff found 1/53-1/69 of Spirostomum amhiguum and 1/70-1/75 

 of Dileptus anser regenerated and Philips showed portions down to 

 1/80 of an amoeba were able to regenerate. In Stentor coeruleus, 

 Lillie and Morgan found pieces as small as 1/27 and 1/64 respect- 

 ively of the original organisms regenerated. Burnside cut 27 speci- 

 mens of this ciliate belonging to a single clone, into two or more 

 parts in such a way that some of the pieces contained a large portion 

 of the nucleus while others a small portion. These fragments re- 

 generated and multiplied, giving rise to 268 individuals. No dimen- 

 sional differences resulted from the different amounts of nuclear 

 material present in the cut specimens. Apparently regulatory pro- 

 cesses took place and in all cases normal size was restored, re- 

 gardless of the amount of the nuclear material in ancestral pieces. 

 Thus biotypes of diverse sizes are not produced by causing inequali- 

 ties in the proportions of nuclear material in different individuals. 



Information on regeneration in individuals in division or encyst- 

 ment is incomplete. While Calkins observed that the regenerative 

 reaction in Uronychia was slowed down in late division stages, in 

 Stentor and Paramecium other investigators found no difference 

 in regeneration between vegetative individuals and those under- 

 going fission. 



