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200 400 600 800 



NUMBER OF CELLS AT TIME OF ORIGINATION 



1000 



Figure 14. Sector origination rate per cell vs. cell number in the shell- 

 forming gland of Mytilus. The ratios of perimeters to sector widths for 

 420 sectors were arranged in rank order, and groups of 30 were formed. 

 The difference between the lowest and highest ratio in a group is an 

 estimate of the number of cells in which the 30 sectors originated. 

 These estimates were converted to sectors per cell and plotted vs. the 

 median ratios of the corresponding groups, that is, the median cell num- 

 ber for each 30 sectors. The dotted hne region may show an artifact 

 caused by compound sectors of common origin. 



Atwood: I don't think so. I feel very strongly about that point. 



Auerbach: But couldn't there be some kind of competition in grow- 

 ing out, which would discriminate against the narrower ones? 



Atwood: There could be, but generally speaking, there is not, because 

 if you look at the cases where you have the two sectors very close 

 together, then the increment of width of the intervening white part 

 and that of the sector itself is similar. We have not any special evidence 

 of differential growth rate, at least not for most of them. 



Of course, these are really approximations because, as the shell 

 grows, it changes shape, so there is a rubber-sheet distortion. Certain 

 parts get disproportionately wider and others do not. But this does 



