4$ J- MCA. KATKR AND R. D. BURROUGHS. 



To carry this further, series F was started. Fi (six culture?) 

 was seeded from A I on the sixtieth day after that series was seeded ; 

 F2 (six cultures) was seeded from 2 on the eighteenth day; 

 7*3 (six cultures) was seeded from active ones that were ten days 

 removed from newly encysted individuals; and 7*4 (eighteen 

 cultures) was seeded from active ones excysted only five days 

 previously. 



On the third day all cultures were excellent, with the exception 

 of Fi, which were in fair condition. No cysts had appeared in 

 the first three series, but some were present in the cultures of Fj. 

 By the seventh day Fi and F2 were in excellent condition, with 

 no cysts while the cultures of F3 were in good condition with 

 some cysts and F4 was only fair with many cysts. Examination 

 on the nineteenth day revealed the fact that the cultures of all 

 four series were very poor. A very few cysts had appeared in 

 several of the cultures of Fi ; there were none at all in F2 while F3 

 and F4 contained many cysts. 



It is now clearly seen that the longer encystment is prevented, 

 whether by low temperature (Fi) or by long continued transfer 

 (F2), the less is the tendency to encyst, and if this procedure be 

 sufficiently extended the ability to encyst is entirely lost. The 

 morphological basis for this is given in the next section. 



It was determined to repeat the work of D and E, using newly 

 excysted forms. Accordingly twelve cultures were seeded and 

 six of these had their medium replaced every day, the others were 

 kept as controls. In two days the controls were quite generally 

 encysted while those filtered every day showed no cysts until the 

 third day. . 



Some cultures of the types found in F2 and F| were filtered and 

 the medium replaced with fluid taken from cultures that had 

 entirely encysted. In the former no encystment whatever 

 occurred, while in the latter it was slowed up, usually for about 

 one day, as compared to the controls. When first placed in such 

 a medium, we were surprised to find that division was apparently- 

 stimulated, as examination under the microscope would reveal a 

 great many late division stages, which are the only ones that 

 can be detected in living material. Although encystment was 

 slightly deferred in the case of those of type F4, the cultures 



