228 CHARLES ZELENY 



because the numbdr of fertile eggs is too small to give an ex- 

 pectation of mutation in every individual. Among males, how- 

 ever, the number of spermatozoa is sufficiently large to give an 

 expectation of several mutations in each individual male in case 

 a considerable percentage of the spemiatozoa are made effective 

 in fertilization. It may be possible to test such a hypothesis 

 by mating a single male to a large number of females. 



11. At what time in the life-cycle do the germinal changes appear? 

 In case a mutation occurs after the last gonial division, only a 

 single affected ovum or two affected spermatozoa are to be ex- 

 pected. If it occurs before the last gonial division, the number of 

 affected eggs or spermatozoa is greater the earlier the appearance 

 of the change. Therefore, it may be argued that the number of 

 mutant individuals appearing at one time is a rough indicator 

 of the period at which the mutation occurred. This procedure is 

 obviously subject to two sources of error. In the first place, it 

 is possible that there may be two independent mutations appear- 

 ing in the same generation. Since the number of individuals 

 appearing in a single bottle rarely exceeded 300 while the rate of 

 appearance of mutations is once in 1500, the chance that two 

 independent mutations will appear in the same bottle is not more 

 than one in five squared or one in twenty-five. The second 

 source of error is more serious. A single adult mutant may be 

 merely the remnant of several mutant sex-cells. What appears 

 to be a mutation appearing late in gonial history may in realit\' 

 have come much earher. 



In general it may be argued that the simultaneous appearanc.^ 

 of two or more individuals indicates with a fair degree of cer- 

 tainty that the germinal change occurred early in oogenesis or 

 spermatogenesis. The appearance of a single mutant individual, 

 on the other hand, does not establish the occurrence of the change 

 as coming late in gonial history. It merely makes it improbable 

 that the change came near the beginning of that period. 



Of the forty-four separate mutations which are suitable for the 

 present purpose and which appeared in the stock bottles, thirty- 

 nine came as single individuals and only five as more than one 

 individual. In figure 3 the dots surrounded by circles represent 



