136 J. A. DAWSON 



4. A twin at first apparently fused in the typical manner pulls 

 apart to give two single animals normal except for dorsal spines. 



It is noteworthy that the miscible condition is soon lost by the 

 single animals derived from twins, since it has been observed in- 

 variably that such animals separate during fission in the same 

 manner as the single animal not of twin origin; i.e., there is an 

 almost direct severing of the daughter cells with no pulling out 

 of a protoplasmic strand. Since in the case of both twins and 

 single animals derived from twins the environment was the same, 

 it is clear that the environment, though probably the chief fac- 

 tor in bringing about the miscible condition of the protoplasm 

 necessary for the formation of the twin animal, is not responsible 

 for the continuance of this condition. In other words, the 'mis- 

 cible' condition of the protoplasm persists as long as the twin 

 strain persists, and disappears as soon as the components of the 

 twin separate. 



The complete dual nature of the double animal or twin cannot 

 be too strongly emphasized. All of the external structures found 

 in single animals have been duplicated in each component of a 

 twin except in the region of fusion. This duality was not limited 

 to external features alone, but it has been found almost invari- 

 ably that the internal structures of each component were not 

 visibly affected by the union. In a few exceptional cases, how- 

 ever, a certain amount of macronuclear fusion has been observed 

 (fig. 22). Animals in this condition do not survive long and 

 there is no evidence to show that such fusion plays any signifi- 

 cant part in the life-history of the twin animal. As in the single 

 animals, here also a certain amount of fragmentation of the 

 macronuclei has been observed (figs. 17, 18, and 20). The usual 

 condition of the macronuclei is shown in figures 15 and 16. 



4. EXPERIMENTS ON PEDIGREED 'TWIN' CULTURES 



a. History of culture AT {series I) 



On July 28, 1917, from the four progeny of one of the twins 

 isolated on the previous day, the four lines of the main culture 

 of twins (AT) were begun. This twin culture was carried by 



