220 Cytoplasm as Seat of Genetic Properties 



the yeast-like symbionts of many insects, as detailed in Buchner, 1953. ) 



For the evaluation of these basic facts a number of details are 

 important. In many respects the active factor behaves like a virus. 

 Inactivation and filtration experiments reveal a size on the order of that 

 of a virus. By continuous dilution of an extract a concentration can be 

 reached which is no longer infective. In this case the individual par- 

 ticles behave independently, since the logarithm of the percentage 

 of resistant flies varies linearly with the inverse of dilution. This per- 

 mits an exact assay of infection results and work with arbitrary units 

 of infection. A study of the incubation time revealed a curve which 

 indicated an exponential multiplication of the particles, until death 

 occurs. This incubation time is dependent upon external and internal 

 factors. A further study of the progress of infection in the fly yielded 

 results closely paralleling those for virus infections. 



The next important point is that flies sensitive by birth yield a 

 constant amount of virus during their whole imaginal life, but this 

 amount varies according to the parent from whom the sensitivity 

 comes. If sensitivity comes from the father, the yield is about 100,000 

 units, which is the same as after a one-unit injection. This shows that 

 the spermatozoon contains a small number of particles. If the sensi- 

 tivity comes from the mother, the virus content is much smaller, a few 

 thousand to 30,000 units. This could be due to waste of some indis- 

 pensable material in the egg, whatever this means. The facts thus far 

 reported fit the idea of an infective virus. 



We come now to the genetic features of the case. A female made 

 sensitive by injection always produces some sensitive offspring from 

 which a pure sensitive line can be derived. A male made sensitive 

 does not transmit this, but the sons of sensitive females transmit sensi- 

 tivity to a part of their offspring. This means, then, that in females, 

 but not in males, acquired sensitivity always enters the germ track. 

 Further, a fly which has inherited sensitivity from the father alone 

 (who had a sensitive mother) behaves like an injected one: if it is a 

 male it is resistant; if a female, some of its offspring are sensitive. 



There are indications that mutation of the virus particle can take 

 place. A line was isolated in which sons of sensitive females hardly 

 transmitted the sensitivity, and this property remained constant. If a 

 mixture of both types was administered to a fly, only one or the other 

 type entered, or was found in, the germ line. Such preferences are, 

 however, known for genuine viruses. Another rather complex case of 

 behavior is attributed to mutation of the infective particles, but on 



