392 



PRINCIPLES OF EMBRYOLOGY 



individual involved. The classical example is the virus-hke particle in the 

 King Edw^ard race of potatoes, w^hich has little effect in that stock but 

 which, when transferred to other races of potato by grafting, produces 

 the symptoms of a severe virus disease (Salaman and Le Pelley 1930). In 

 other cases such variation in effect is less in evidence, but it seems likely 

 that careful search would always reveal some degree of variability of this 

 kind. 



2. True plasmagenes 



One can pass by more or less insensible gradations from cases in which 

 infection is easy and the infecting particle obviously foreign, to the other 



®® ®® 



KILUH SENSITIVE 



HiLten KiufR 



Figure 18.4 



Transfer of kappa when conjugation lasts abnormally long. The rate of 

 multiplication of the transferred kappa has been exaggerated for diagram- 

 matic purposes. (From Beadle 1948, after Sonnebom and others.) 



end of the range at which infection cannot be defmitely demonstrated to 

 occur, and the plasmagenes appear to be normal constituents of the organ- 

 ism. Particles coming at the latter end of the range may be considered as 

 true plasmagenes. Most of the cases knov^ni are from the plant world 

 (reviewed in Caspari 1948). Perhaps the best investigated are the factors 

 which are clearly inherited through the cytoplasm in crosses between 

 different races of the willow herb, Epilobium (Michaehs 195 1). Another 

 example is provided by the cytoplasmic factors causing male sterility in 

 crosses between races of a number of different species of plants (e.g. flax, 

 maize, etc.). 



Evidence for such factors in higher animals is exceedingly rare. The 

 case which perhaps seems most likely to fall into the category is that de- 

 scribed by Laven (1953) in mosquitoes of the genus Culex. He made 

 reciprocal crosses between a number of races of the species-group Culex 

 pipens, and found that they fell into three groups; between the members 



