AN AMICRONUCLEATE OXYTRICHA. II 135 



As the separation became more marked the ventral surfaces 

 of both animals rested on the bottom, as shown in figure 19. 

 The ensuing tug-of-war caused the separation of the twin, usu- 

 ally in half an hour to an hour. During this process there always 

 occurred a pulling out of the protoplasm into a strand of varying 

 diameter, but never so fine as that observed in the reproduction 

 of a twin by fission. Figure 20 shows a twin in which such a 

 connecting strand is present. After the break occurred, the re- 

 mains of the strand could be seen for some time on each indi- 

 vidual as a posterior dorsal spine (fig. 21) entirely comparable 

 to that described by Jennings ('08, p. 625). This spine, however, 

 is invariably completely resorbed within an hour or so and has 

 never in the history of any of the strains been transmitted at 

 fission to a daughter animal. 



When the amount of anterior separation of a twin was some- 

 what greater than in the case just described, the fission which 

 followed resulted in the direct separation of two single animals 

 from the separated anterior ends of the present twin, while the 

 posterior portion remained a twin animal. In rare cases a twin, 

 apparently normal, would pull apart before fission occurred, thus 

 giving two single animals. In carrying pedigrees of twin animals, 

 single animals which had arisen in this way are always regarded 

 as of the same generation as the twin from which they came. 



The methods of multiplication of twins may be summarized 

 as follows : 



1. A twin divides and thus gives rise to two typical twins (figs. 

 15 and 16). 



2. A twin, somewhat separated anteriorly, divides, producing, 

 from the anterior end, a twin widely separated anteriorly — and 

 from the posterior end, a typical twin. The widely separated 

 daughter twin pulls apart to give two single animals, typical in 

 every respect except for the dorsal spines which are invariably 

 soon resorbed and are never transmitted to the next generation 

 (figs. 17, 18, 19, 20, and 21). 



3. A twin with a marked separation of the components at 

 the anterior end divides, resulting in two single typical animals 

 from the anterior end and a twin, usually with only slight 

 anterior separation, from the posterior end. 



