244 PROTOZOA 



the natural source of a strain is known, its identification is seem- 

 ingly limited to but few possibilities. 



2. Karyotypes. Chen (1946b) reported that the chromosomes 

 of varieties 2 and 4 are thin and short while those of variety 3 are 

 larger and much longer. However, similar differences distinguish 

 strains of variety 6 collected from different parts of Europe, Eng- 

 land and Czechoslovakia. The usefulness of this criterion is ham- 

 pered not only by the possibility of differences within a variety, 

 but also by the fact that one must obtain conjugants to examine 

 the chromosomes satisfactorily. Late prophase or prometaphase 

 of the first meiotic division is the stage employed. 



3. Number of mating types. As stated above, varieties differ 

 as to whether they have 2, 4, or 8 mating types. With regard to 

 the usefulness of this criterion, the pertinent question is: Can 

 the number of mating types in a variety be readily ascertained? 

 Success in answering this question depends upon finding at least 

 two different mating types in the variety, for Jennings ( 1941 ) 

 reports that all types of a variety can be obtained among the 

 sexually produced descendants of any two. The most reliable way 

 of finding two types is to make a number of collections from 

 nature in the same region. If only one type is found in nature, it 

 is still possible to obtain all types, but this depends upon the oc- 

 currence of a rare event: self-differentiation of an additional 

 mating type in a culture previously pure for one mating type. 

 According to Jennings (1941), this happens on the average once 

 in about 2000 culture-days. Once a culture yields a second type, 

 conjugation between the two types can yield all the others in the 

 variety. Lee (1949) reported that exposure to high doses of x-rays 

 induced selling in a previously pure culture. He suggests that 

 the irradiation induced self-differentiation of the sort that occurs 

 rarely without irradiation. If this proves repeatable and general, 

 a method of finding all types in a variety would be in hand. 

 However, the process of doing this would still take much time 

 and labor because immaturity is apparently a regular occurrence 

 alter conjugation, and it may last a long time. Incidentally the 

 difficulty of getting all types from one (without Lee's technique ) 

 is attested by the fact that in years of observation the one collec- 



