152 J. A. DAWSON 



have occurred with predictable regularity throughout the whole 

 period of the study. The forms described are, indeed, atypical, 

 judged by our present knowledge of the morphology and life- 

 history of hypotrichous ciliates, but one may doubt whether this 

 is broad enough to permit one to consider previously unknown 

 phenomena as abnormal. Certainly, this race from every point 

 of view is vigorous. The cannibals show at least as great repro- 

 ductive ability, while the twins also, although apparently not 

 able to compete with single individuals under the conditions of 

 the experiment, are fully as vigorous. 



Since the double animals are always eliminated in a short 

 time when allowed to breed in a mass culture, the permanent 

 establishment of such a form in nature is highly improbable. 

 It is therefore considered that such twin formation is but one 

 feature of the life-history of this amicronucleate race; — the 

 result of an attempt to conjugate which is abortive probably 

 because the organisms lack the nuclear constituents in a form 

 for the carrying out of this process — it having been demon- 

 strated (part 1), it is believed, that this race does not possess 

 idiochromatin morphologically segregated as micronuclei. 



7. SUMMARY 



1. In cultures of an amicronucleate race of Oxytricha hymen- 

 ostoma (part I), under conditions similar to those in which syn- 

 gamy usually takes place in hypotrichous forms, there is a strong 

 tendency for the formation of double animals or 'twins' by plas- 

 togamic dorsal fusion. 



2. Twins, morphologically, have all the structures possessed 

 by two single animals. 



3. Twins reproduce giving, a) two pairs of twins exactly similar 

 to the parent; h) from the anterior portion of the parent, a twin 

 which pulls apart to produce two single animals, typical, except 

 for the temporary presence of dorsal spines; from the posterior 

 portion of the parent, a typical twin; c) from the anterior portion 

 of the parent, two typical single animals; from the posterior 

 portion, a typical twin. The components of a twin may pull 



