226 PROTOZOOLOGY 



normal form in 3 or 4 generations and that three factors appeared to 

 determine the character and number of teeth: namely, the size of the 

 mouth, the number and arrangement of teeth in the parent, and 

 "something in the constitution of the clone (its genotype) which 

 tends toward the production of a mouth of a certain size, with teeth 

 of a certain form, arrangement, and number." 



Races or strains have been recognized in almost all intensively 

 studied Protozoa. For example, Ujihara (1914) and Dobell and Jepps 

 (1918) noticed five races in Entamoeba histolytica on the basis of dif- 

 ferences in the size of cysts. Spector (1936) distinguished two races in 

 the trophozoite of this amoeba. The large strain was found to be 

 pathogenic to kittens, but the small strain was not. Meleney and 

 Frye (1933, 1935) and Frye and Meleney (1939) also hold that there 

 is a small race in Entamoeba histolytica which has a weak capacity for 

 invading the intestinal wall and not pathogenic to man. Sapiro, 

 Hakansson and Louttit (1942) similarly notice two races which can 

 be distinguished by the diameters of cysts, the division line being 

 10/x and 9m in living and balsam-mounted specimens respectively. 

 The race with large cysts gives rise to trophozoites which are more 

 actively motile, ingest erythrocytes, and culture easily, is patho- 

 genic to man and kitten, while the race with small cysts develops 

 into less actively motile amoebae which do not ingest erythrocytes 

 and are difficult to culture, is not pathogenic to hosts, thus not being 

 histozoic. It is interesting to note, however, that Cleveland and 

 Sanders (1930) found the diameter of the cysts produced in a pure- 

 line culture of this sarcodinan, which had originated in a single cyst, 

 varied from 7 to 23m- Furthermore, the small race of Frye and 

 Meleney mentioned above was later found by Meleney and Zucker- 

 man (1948) to give rise to larger forms in culture, which led the last 

 two observers to consider that the size range of the strains of this 

 amoeba is a characteristic which may change from small to large or 

 vice versa under different environmental conditions. 



Investigations by Boyd and his co-workers and others show that 

 the species of Plasmodium appear to be composed of many strains 

 which vary in diverse physiological characters. In an extended study 

 on Trypanosoma lewisi, Taliaferro (1921-1926) found that this flagel- 

 late multiplies only during the first ten days in the blood of a rat after 

 inoculation, after which the organisms do not reproduce. In the adult 

 trypanosomes, the variability for total length in a population is about 

 3 per cent. Inoculation of the same pure line into different rats some- 

 times brings about small but significant differences in the mean size 

 and passage through a rat-flea generally results in a significant vari- 



