Crocheting: shells int eh round

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For reference, I'm using this pattern: http://www.angelfire.com/va3/heartfeltangels/littlestarafghan.html

It's crocheting in the round using shells. What I don't understand is on the second row there is more than one chain in each space to choose from and I don't know which to work in when doing my shells.

And although I haven't gotten that far, I'm confused as to which chain to work in following the pattern. The first chain in the middle of each shell or the second?