AN AMICRONUCLEATE OXYTRICHA. II 153 



apart completely, producing two single animals, typical, except 

 for the temporary presence of dorsal spines. 



4. Twins may form, a) from normal strains in which no such 

 forms have previously occurred, i.e., since isolation of original 

 ancestor; h) from descendants of single animals arising from twins; 

 c) from progeny of cannibal animals. 



5. Favorable environmental conditions are necessary for the 

 continued existence of twin forms; i.e., in stock and mass cultures 

 in competition with single animals twins do not survive. 



6. By selection a striking increase in the percentage of twins, 

 produced in a pedigreed culture from a single twin animal, has 

 been obtained. 



7. The division rate of twin races is similar to that of normal 

 single animals. 



8. The miscible condition of the cytoplasm which leads to the 

 formation of a twin animal continues as long as the animal 

 lives; i.e., this condition is transmitted from parent twin animal 

 to its progeny. In the case of single animals derived from twins 

 and kept under identical environmental conditions, this miscible 

 condition is quickly lost. 



9. If twin animals are bred under favorable conditions (see 

 5 above) the union is usually permanent. A pedigreed strain 

 (series I) of twins has been bred for one hundred and two genera- 

 tions, and it is believed that if suitable environmental conditions 

 could be secured, the twin strains could be bred indefinitely. 



10. As pairing, cannibalism (part I), and twin formation occur 

 among animals in a similar physiological condition, it is believed 

 that these three phenomena are but expressions of an abortive 

 attempt to undergo the process of syngamy, which fails probably 

 because the organism lacks the nuclear constituents in the 

 proper form for the completion of this phenomenon — -it having 

 been demonstrated (part 1), it is believed, that this race 

 does not possess idiochromatin morphologically segregated as 

 micronuclei. 



11. The inability of* the organism to consummate syngamy 

 clearly has no effect on the viability of the race. 



