EFFECTS OF THYROID ON PARAMAECIUM ^ 533 



sion period after a life history of 420 generations. The con- 

 trol medium used was one-half strength standard hay infusion, 

 prepared by boiling 1 gram of hay for ten minutes in 100 cc. of 

 tap water and allowing it to stand exposed to the air for twenty- 

 four hours, when it was mixed with an equal quantity of boiled 

 tap water. The life history of this race as shown in the daily 

 division rate averaged by ten day periods is given in figure 1. 



Race B. Paramaecium caudatum. Descended from an indi- 

 vidual isolated July 2, 1914, from a stock culture of the Marine 

 Biological Laboratory. This race was maintained as a pedigreed 

 culture until October 18, when a pair of conjugants was isolated 

 from an epidemic of conjugation which had been induced in 

 stock at about the 150th generation. From an indi\ddual re- 

 sulting from the third division after the ex-conjugants had sepa- 

 rated, or in other words, when reorganization was completed, a 

 new line B' was established. 



Line B'. This line was maintained in a pedigreed culture 

 until January 6, 1915, when it died out in a depression period, 

 81 generations after conjugation and 230 (approximately) after 

 the original isolation. An experiment with boiled thyroid was 

 in progress at this time, and from this hne after thirty days of 

 thyroid treatment a new line B" was established. 



Line B". This line was maintained similarly until May 31, 

 1915, when it died out in transportation from New York. It 

 had been maintained for 159 generations after thyroid feeding 

 was discontinued and 460 after the original isolation of the race. 

 The control medium employed for all these lines was prepared 

 by boiling approximately 0.5 gram of hay seed in 50 cc. of spring 

 water, filtering and using it after cooling. Figure 2 shows the 

 life history of this race by ten day periods. 



Race C. Paramaecium caudatum. On July 27, 1915, a con- 

 jugating pair was isolated from an epidemic of conjugation in- 

 duced in stock from the Zoological Laboratory of Smith College. 

 Two divisions after separation one of the quadrants was isolated. 

 From this individual a pedigreed culture was established and 

 has been maintained up to the date of writing when it stands in 

 the 172nd generation. The control medium used for this race 



