PROBLEMS OF MEASUREMENT OF MUTATION RATES 47 



strip of tissue running around the edge. It is obvious from the relations 

 of these structures that in order to form a pigmented sector the gland 

 must have a little island of cells in it here that are producing pig- 

 mented shell, whereas the rest are producing nonpigmented shell. 



Then, you might infer that if there is no differential growth rate of 

 the cells that are producing pigment as compared with those that are 

 not, the time of origin of a sector from a single cell could be deduced 

 simply by measuring the width of the sector out here where it is wide 

 enough to measure, and measuring the perimeter of the shell. The 

 ratio of the sector width to the perimeter immediately gives the number 

 of cells in the gland at the time of origin. For example, if this is 1 

 per cent of the perimeter, then we had 100 cells present when this 

 sector originated as a single cell. 



Auerbach: Why do they always go back to the umbo? 



Atwood: When the mussel first sits down after its free-swimming 

 larval stage, this represents the point at which the earliest shell is 

 formed, and from this point it grows, so now, if you establish early a 

 little island of cells that form pigmented tissue and they maintain 

 their spatial relation to the other cells, this sector has to come out 

 from the origin. 



Dobzhansky: Does the sector always reach the umbo? 



Atwood: The shell-forming gland is a columnar epithelium that has 

 cells 4 micra in width, so that one has to accumulate a fair number 

 of these cells before the sector is big enough to see. Therefore, the 

 sectors seem to vanish as they recede to the umbo. But we know that 

 the sector really must originate from somewhere down here. If a sector 

 were to originate out here where the animal is large and then, let's 

 say, the perimeter doubles, you would now have on the average two 

 cells instead of one. You would never detect it. Therefore, any sectors 

 originating far out are going to be undetectable until a lot of growth 

 occurs. But awareness of this fact has enabled the deductions that I 

 am about to make, from having measured such sectors. 



You see, the reason for doing this was because it seemed to afford 

 an analogous situation to the absence of age effect, as you will see. If 

 we were to take this shell and grind it up and measure how much 

 pigment was in it, this would be analogous to taking blood and seeing 

 what proportion of cells are exceptional. However, instead of that, we 

 have a permanent record of everything that happened in this shell, up 

 to a certain size. 



The number of sectors per shell is better correlated between right 

 and left halves of the same animal than among different animals, but 



