HEREDITY AND VARIATIONS IN PROTOZOA 



577 



nucleate condition persisting for many generations but ultimately 

 ending in death since failure to conjugate is characteristic of such 

 races. These are evidently not cases of mutation but temporary 

 abnormalities resulting from imperfect reorganization. 



Variations following from Mendelian segregation have been 

 repeatedly described among higher animals and plants; with Proto- 

 zoa they are limited to one single case, described by Pascher (1916, 

 191S). Two different but not fully identified species of the phyto- 

 flagellate Chlamydomonas were cultivated in rich cultures. One 

 of these (A) was characterized by a pyriform body, lateral, single 

 chromatophore with lateral pyrenoid, two rather long, equal flagella, 

 and a narrow dash-like stigma (Fig. 236). The other species (B) 

 was more spherical and was characterized by a basal, single, chroma- 

 tophore with basal pyrenoid; rather short flagella and a swollen 

 stigma (Fig. 236, B). Both types reproduce by longitudinal 

 division, and under appropriate conditions both types form iso- 

 gametes. The differences in the two sets of gametes are of the 

 same type as the difl'erences between .1 and B (Fig. A', B'). Fer- 

 tilization is total (merogamic) and there is but slight difference 

 manifested by the two types. Gametes of .1 shed their thick mem- 

 branes on fusion, the old membranes remaining attached to the 

 zygote as appendages (Ohrchen) ; gametes of B have a delicate 

 membrane which is retained on fusion. The zygotes of the two 

 species are also different; that of A is covered by a membrane 

 ornamented by many conical warts {CAA, CBB); that of B is 

 covered by several layers of smooth membranes. On germination of 

 the zygotes 4 individual swarmers with the characteristics of their 

 species emerge from both types of zygotes. Reduction in number of 

 chromosomes evidently occurs in the zygote so that the ordinary 

 vegetative individuals are haploid, the zygote alone diploid, and the 

 4 resulting swarmers are likewise haploid (zygotic reduction). 



Pascher succeeded in crossing gametes of A and B, the hetero- 

 zygotes ha^ing characters intermediate between those of the zygotes 

 of A and B (Fig. C,A,B; C,B,B). On germination these hetero- 

 zygotes gave rise to 4 different kinds of swarmers; 1 was like A, 

 1 like B and 2 were mixed as indicated in Fig. 236, and showm in 

 the following table from Pascher (1918, p. 166). 



37 



