Heredity in Protozoa 599 



dently in an unhealthy condition. Investigation showed that the 

 wrong sort of bacteria had multipHed in the culture fluid last 

 made, making it opaque and sirupy. All the specimens (for 

 various other experiments) that had been placed in this fluid were 

 unhealthy or dying. This multiplication of injurious bacteria 

 in culture fluid made in the usual way, is a not uncommon and 

 most disastrous occurrence. To it we shall return in another 

 connection. 



The two sister individuals (one with the spine, the other without) 

 were transferred to clean water, and later to new culture fluid. 

 They were still living May 21, three days after the last preceding 

 fission. But on the morning of May 22 I found, to my great 

 regret, that the individual with the spine had died. Its sister 

 recovered and propagated the race for many generations, of which, 

 we shall have to speak in our account of the hereditary tendency 

 to remain connected after fission. 



The last individual bearing the spine was designated 

 ^1.2.1.2.1.2. 1.2.2.2.2.1. 1.1.2.1.1.2.1. i.i_ These exponents show to which individ- 

 ual the spine passed at each division — (^) indicating the anterior 

 individual, (^) the posterior one. The spine was traced through 

 twenty-one generations (the first generation not having the spine). 

 Fig. 4 gives a diagram of the entire history of this structure. 



In this history of a localized new structure for twenty-one gen- 

 erations, certain general relations appear, which we will here set 

 forth, though a full discussion of their significance will be reserved 

 till other cases have been considered. 



2 General Relations and Processes Shown in the History of the 



New Structure 



I The new structure was transmitted in each generation to but 

 one individual. Thus, in the sixth generation there were thirty- 

 two individuals, with but one bearing the spine (see Fig. 6); in the 

 eleventh generation, out of 1024 individuals, but one had the spine; 

 in the twenty-second generation the spine was found on but one 

 individual out of 2,097,152. 



Furthermore, the spine occupied a definite place in the series 

 of individuals produced. As we have seen, and shall see farther, 



