206 R. RUGGLES GATES. 



means an improbable one. It does not assume that the chromo- 

 somes which underwent the change were alike, but merely that 

 they were more nearly alike than the others of the series. That 

 the chromosomes of the x series are actually differentiated 

 there are many lines of evidence to show. One of the most 

 recent is the fact, ascertained by Doncaster and Gray, 1 that in 

 certain echinodern crosses particular chromosomes swell up and 

 form vesicles in the strange cytoplasm of the egg or of another 

 species while other chromosomes exhibit no such effect. On 

 the other hand, the materials of the chromosomes obviously 

 possess many similarities which probably outweigh their chemi- 

 cal differences. 



It may further be pointed out that if both members instead 

 of one member of a pair of chromosomes underwent a change, 

 say in a pollen mother cell, the only difference would be that 

 four instead of two mutated germ cells would result, each of 

 which might function in producing a mutant. It is almost 

 impossible to determine whether one or both members of a pair 

 of chromosomes underwent the change in any particular in- 

 stance, but in either case the original mutant would be hetero- 

 zygous, though continued inbreeding would produce ultimately 

 a homozygous race, as in the case of (E. rubricalyx. This is 

 probably the history of Nilsson-Ehle's wheats which are dup- 

 licate or triplicate for the red color factor in their grains. 



From the evidence obtained in F 2 and F 3 in crosses of various 

 Swedish wheats having red kernels, with white-grained varieties, 

 Nilsson-Ehle concludes 2 that while the varieties known as 

 Sammet and Grenadier have 3 independent units for red, Extra- 

 Squarehead has only one, since it gave (p. 67) only ratios approxi- 

 mating 3:1. In a later paper 3 continuing this work the same 

 author finds (p. 22) that Swedish Binkel wheat contains two 

 factors for red. From one FZ family he grew 94 FS families, 

 with results which may be tabulated as follows: 



1 Doncaster, L., and Gray, J., 1913, "Cytological Observations on the Early 

 Stages of Segmentation of Echinus Hybrids," Quart. Journ. Micr. Sci., 58: 483- 

 510, pis. 28-29. 



2 Nilsson-Ehle, H., 1909, Kreuzungsunterschungen an Hafer und Weizen. I. 

 Lunds Univ. Arsskrist., N.F., Afd. 2, Bd. 5, Nr. 2, pp. 122. 



3 Nilsson-Ehle, H., 1911, " Kreuzungsuntersuchungen an Hafer und Weizen," 

 II., Lunds Univ. Arsskrift., N.F., Afd. 2, Bd. 7, Nr. 6, pp. 82. 



