232 PROTOZOOLOGY 



of course found. It was found that about half the zygotes gives rise to 

 the two parental combinations (Fig. 97, b, c), while the other half 

 gives rise to FPsD (e) and fpsD (/). 



When B (FpsD) is crossed with D (fPsd) or A (FpSD) is crossed 

 with D (fPsd), only two types of swarmers are also formed from 

 each zygote, and in the case of BxD, eight different combinations 

 are produced, while in the case of AXD, sixteen different combina- 

 tions, which appear in about equal numbers, are formed. Thus these 

 four factors or characters show independent assortment during divi- 

 sions of the zygote. 



a b c d e ( 



Fig. 97. a, b. Polytoma uvella. a, Form A; b, P^orm B. 

 c, d. P. pascheri. c, Form C; d, Form D. 

 e, f. Crosses between Forms B and C. (Moewus) 



Furthermore, Moewus noticed that certain other characters ap- 

 peared to be linked with some of the four characters mentioned 

 above. For example, the length of flagella, if it is under control of a 

 factor, is linked on the same chromosome with the size-controlling 

 factors (D, d), for large individuals have invariably long flagella 

 and small individuals short flagella. During the experiments to de- 

 termine this linkage, it was found that crossing over occurs between 

 two entire chromosomes that are undergoing synapsis. 



In certain races of Polytoma pascheri and Chlamydomonas euga- 

 metos, the sexual fusion takes place between members of different 

 clones only. The zygote gives rise as was stated before to four swarm- 

 ers by two divisions, which are evenly divided between the two 

 sexes, which shows that the sex-determining factors are lodged in a 

 single chromosome pair. In a cross between Chlamydomonas para- 

 doxa and C. pseudoparadoxa, both of which produce only one type of 

 gamete in a clone, the majority of the zygotes yield four clones, two 



