VARIATION AND HEREDITY 231 



three years. Nothing is known as to how such strains arise, though 

 some workers suggest mutational change. 



In sexual reproduction, the nuclei of two individuals participate 

 in producing new combinations which would naturally bring about 

 diverse genetic constitutions. The new combination is accomplished 

 either by sexual fusion in Sarcodina, Mastigophora, and Sporozoa, 

 or by conjugation in Euciliata and Suctoria. 



The genetics of sexual fusion is only known in a few forms. Perhaps 

 the most complete information was obtained by Moewus through 

 his extended studies of certain Phytomonadina. In Polytoma (p. 

 281), Chlamydomonas (p. 276), and allied forms, the motile indi- 

 viduals are usually haploid. Two such individuals (gametes) fuse 

 with each other and produce a diploid zygote which encysts. The 

 zygote later undergoes at least two divisions within the cyst wall, in 

 the first division of which chromosome reduction takes place. These 

 swarmers when set free become trophozoites and multiply asexually 

 by division for many generations, the descendants of each s warmer 

 giving rise to a clone. 



Moewus (1935) demonstrated the segregation and independent as- 

 sortment of factors by hybridization of Polytoma. He used two va- 

 rieties each of two species: P. uvella and P. pascheri, both of which 

 possess 8 haploid chromosomes. Their constitutions were as follows: 



P. uvella 



Form A: Oval (F), without papilla (p), with stigma (S), large (D) 



(Fig. 97, a). 

 Form B: Oval (F), without papilla (p), without stigma (s), large (D) 



(Fig. 97, b). 



P. pascheri 



Form C: Pyriform (f), with papilla (P), without stigma (s), large 



(D) (Fig. 97, c). 

 Form D: Pyriform (f), with papilla (P), without stigma (s), small 



(d) (Fig. 97, d). 



Thus six different crosses were possible from the four pairs of 

 characters. When A (FpSD) and B (FpsD) fuse, the zygote divides 

 into four swarmers, two swarmers have stigma (S), and the other 

 two lack this cell organ, which indicates the occurrence of segrega- 

 tion of the two characters (S, s) during the reduction division. When 

 B (FpsD) is crossed with C (fPsD), thus differing in two pairs of 

 characters, two swarmers possess one combination or type and the 

 other two another combination. Different pairs of combinations are 



