48 MUTATIONS 



the positions of the sectors in right and left halves of the same animal 

 are independent. 



Lederberg: What is the source of the shell-to-shell variance, which 

 is responsible for the apparent correlation? 



Atwood: Simply that these two halves of the same animal have the 

 same rates of sector origination. 



Lederberg: But then different shells have different rates of origina- 

 tion. 



Atwood: That's right; different individuals do have different rates 

 of origination. 



Auerbach: Why do you think they should have different rates be- 

 tween animals? 



Atwood: We have no idea what the genetic basis for the sectoring is 

 or why it has different rates. 



Cotterman: You're talking now about the small minority of shells 

 that show some variegation. Are the rest all uniformly blue? 



Atwood: Most of them are uniformly blue, but there is another class 

 that is blue, basically, but with a few colorless sectors. These are hard 

 to discover because the shell gland, except in very small individuals, 

 is thick enough so that an island of nonpigment-forming cells in it 

 will make a white sector imbedded in blue. This is much harder to see 

 than the reverse situation. I don't know the frequency of these except 

 that it is higher than the frequency of basically albinos with a few 

 blue sectors. Some individuals are not easy to place in either category. 



Occasionally, you find one that has thousands of very narrow sectors 

 that all seem to originate at about the same time. I am not considering 

 these in the present discussion. 



Lederberg: In view of the variation from individual to individual, 

 are you going to question the regularity of the cell lineages, and the 

 number of stripes you may get, which may simply reflect how dis- 

 persed the cells from a single mutant clone may be? There is no 

 certainty that different stripes come from really independent clones. 



Atwood: You mean that when there are several sectors, these may 

 have had a common origin, but somehow the cells became dispersed. 



The point I'm going to get at is that the rate of origination is high 

 early, and declines later. A dispersal such as you suggest would mean 

 that the true originations were even earlier than it would appear from 

 any of the resultant sectors. 



Auerbach: But if you can't discover the ones which happened late, 

 these might have happened more frequently — 



Atwood: That problem has a simple solution; we can find the rates 



