VARIATION AND HEREDITY 183 



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 



E 



Fig. 87. a, b. Polytoma uvella. a, Form A; b, Form B. 

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

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



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

 the two parental combinations (Fig. 87, h, c), while the other half 

 gives rise to FPsD (Fig. 87, e) and fpsD (Fig. 87,/). 



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. 



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. 



