PLASMAGENES 393 



of a group, crossing led to offspring whichever race provided, the female, 

 while when the crosses were made between races of different groups, 

 they only succeeded in one of the two possible ways. Thus the cross 

 between the O and H races gave offspring only when the female belonged 

 to the latter. Further analysis showed that the infertility of the opposite 

 cross is due to a factor carried in the H sperm, which inhibits the develop- 

 ment of the embryo when it gets into an egg with O cytoplasm. By 

 backcrossing of the hybrid females to O males for several generations, 

 one obtains males which will contain an almost completely O genotype; 

 nevertheless they continue to give sperm which lead to a failure of deve- 

 lopment of the O eggs. It seems that this must be due to a cytoplasmic 

 factor which came from the H female in the original interracial cross and 

 which has been carried on by the eggs through subsequent genera- 

 tions, without being influenced by the increasing number of O genes. 

 The nature of the factor is still obscure. It may be similar to the plasma- 

 genes described in plants such as Epilohium. But its distribution is worthy 

 of note. In Culex it distinguishes various geographical races; and within 

 a race which has it, it is quite undetectable until an attempt is made at an 

 interracial cross to a strain which lacks it. In another related genus, a 

 similar factor, or perhaps the same, distinguishes the two species Aedes 

 aegypti and A. alhopictus; and in Aedcs scutellaris there is a distinction be- 

 tween two local races similar to that in Culex. This suggests that we are 

 dealing with a phenomenon which might be compared to the acclima- 

 tisation of some local races to a virus, rather than with a situation which 

 can assist us to understand the origin of tissue differentiation. There are 

 in the hterature a few other cases of persistent cytoplasmic differences 

 between local races (cf Goldschmidt 1938 on Lymantria) but these require 

 further investigation. 



None of the entities in this category of true plasmagenes can yet be 

 seen and there is no direct evidence as to the size of particle involved. 

 The indirect evidence, chiefly from the type of physiological effect which 

 they produce, is usually held to suggest that they are bodies of a gene-like 

 order of complexity. It is not impossible, however, that in the future 

 some of them may turn out to be simpler than has been previously thought. 



3. Visible cytoplasmic particles with genetic continuity 



In many Protozoa, self-duplicating cytoplasmic particles can be seen 

 fairly easily in microscopical preparations stained in the appropriate way 

 (by silver impregnation, for example). There are, for instance, the granules 

 which lie at the base of the ciha with which the body of a ciliate is covered 

 (Faure-Fremiet 1948, Lwoff 1949, 1950, Weisz 1951). These so-called 



