VARIATION AND HEREDITY 167 



experimental hybridization of Protozoa is however very meager. 

 Pascher (1916) succeeded in producing a small number of 

 hybrid zygotes between two species of Chlamydomonas (Fig. 82). 

 The two possessed the following characteristics. Species I: 

 pyriform; without a membrane-papilla; with a delicate mem- 

 brane; flagella about twice the body length; chromatophore and 

 pyrenoid lateral; nucleus central; stigma a narrow streak in the 

 anterior third; with 2 contractile vacuoles (a); division into 4 

 zoospores ; gametes up to 8, narrowed without membrane ; zygote 

 deeply sculptured and without spreading envelope (e). Species 

 II: spherical; with a distinct membrane and a membrane-papilla; 

 chromatophore and pyrenoid posterior; nucleus central; stigma 

 more anterior and fusiform; flagella short (6); division into 4 

 zoospores; gametes ellipsoid, ends rounded; zygote with a smooth 

 but spreading envelope; with discarded gamete membrane (/). 

 The hybrid-zygotes were morphologically intermediate {h, i) 

 between the two parent zygotes. Thirteen zygotes were reared 

 and in five cultures the offspring were either species I or II, two 

 of each four zoospores being similar to those of I and the other 

 two similar to those of II. In the eight cultures, each zygote de- 

 veloped into four different zoospores. Pascher described these 

 four zoospores (j-m), which tended to indicate that for each of 

 several pairs of characters, two zoospores possessed that of I 

 and the other two that of II and that hybridization brought to- 

 gether two diverse sets of determiners in the heterozygote, which 

 became segregated into four new sets of determiners, because 

 of reduction during the formation of zoospores. These zoospores 

 were however less active and abnormal so that they finally died 

 in the culture without further development. Strehlow (1929) at- 

 tempted to produce hybrids from three combinations of species 

 of Chlamydomonas, succeeding in only one. Heterozygotes were 

 obtained from the "positive" strain of C. paradoxa and the 

 "negative" strain of C. hotnjodes (Fig. 83). Germination of the 

 zygotes was however not observed. 



Hybridization between different varieties or different species 

 of the same genus of ciliates, was either unsuccessful or not 

 genetically studied until quite recently. By using different clones 

 of Paramecium aurelia which differed in fission rate, viability, 

 and body length, Sonneborn and Lynch succeeded in following 

 through three or four sexual generations and observed: "Groups 



