PROBLEMS OF MEASUREMENT OF MUTATION RATES 49 



of origination, relative to cell number, for sectors that are all well 

 within the visible range. I threw out about 30 per cent of the visible 

 sectors as being too small to measure. I am purposely taking a certain 

 range of sizes and finding out within that range what is the distribution. 



Benzer: The variegation has been studied in detail in corn and shown 

 to have this kind of basis. 



Atwood: What kind of basis? 



Benzer: Early mutations arising at different times along the de- 

 velopment of the cell line. 



Atwood: I am asking the question, is the rate of origination constant 

 per cell, or not, as the animal grows. The answer is that in this case 

 it is not. You can plot now the number of cells at time of origin 

 versus the (per cell) frequency of origins corresponding to such 

 numbers. It looks like this (Fig. 14) in the particular ones that I 

 measured; 420 sectors. 



Benzer: What is that curve? 



Novick: He dates the things by the number of stripes he sees. This 

 is the number having a given date. 



Benzer: Number per time? 



Novick: Size of the stripe ; so that most, or the majority, of the stripes 

 originate at about 100 cells, and by the time there are 1000 cells, there 

 are almost no more origins. 



Lederberg: What is the evil of picking them up, though, by the en- 

 tire scale? Don't you have some difficulty in seeing them? 



Atwood: There are a lot more beyond 1000 that I excluded purposely 

 to show that these can all be picked up. 



Lederberg: Do you know that you can pick them up if the stripe is 

 1000? 



Atwood: I know there are some more here than I can't pick up. But 

 what you are saying is how do I know this rate of origination won't 

 go up again sometime in the future, when all are too small to see? 



Lederberg: The curve is naturally flat, but you have less and less 

 chance of seeing the events that occur at those stages, because they 

 result in — 



Atwood: But, up to here and even past here, they are all visible. I 

 measured the widths up to here only. If a sector were so narrow that 

 it was invisible, it would be out here somewhere. It would have a 

 number of cells at time of origin greater than 1000. 



Lederberg: You don't think there are any other stripes as wide as the 

 ones that you have recorded that you have failed to record because 

 they were not contrasting enough? 



