PARAMOECIUM POSSESSING EXTRA VACUOLES 323 



as they were on March 15. Conjugation had by this time 

 dropped to less than 0.5 per cent of the population. 



e. Attempts to cross the two races 



One of the most interesting points to be developed in connec- 

 tion with this new race of paramoecium is the effect of crossing 

 with the common form. This is the first time to my knowledge 

 that two forms of protozoa with definite individual characters 

 have been available for crossing. I have made one hundred 

 and fifty attempts to cross the two forms with no success. The 

 difficulties that beset the mating of these two paramoecia are 

 many. Cultures of both races have to be entering into a period 

 of conjugation at the same time. Single individuals from both 

 races must be placed in as small amount of fluid as possible so 

 that they will have every opportunity of coming together. 

 The chances that either or both of the mated paramoecia are 

 in a condition which makes conjugation necessary are small. 

 The matings must be made up fregh at least, every twenty- 

 four hours for when either of the individuals divide there is no 

 way of telling to which race an animal belongs. In spite of these 

 obstacles I am confident that the crossing will ultimately be 

 achieved after sufficient number of matings have been made 

 and perhaps after certain methods of technique have been deter- 

 mined which are not evident at present. 



VII. CYTOLOGY 



My studies on the cytology of these paramoecia are not 

 sufficient as yet to permit a detailed account. There is one 

 micronucleus present (fig. 15) which indicates that this race is a 

 variety of Paramoecium caudatum. In all the preparations 

 made I have had great difficulty in staining the micronucleus 

 and in the majority of cases so far I have been unable to make the 

 stain remain in it. The depression in the macronucleus where 

 the micronucleus usually lies is frequently visible but it appears 

 empty. I do not believe that this appearance is due entirely to 

 faulty technique, as the same methods give excellent results 

 when used on the common race. 



