Seel, do trials try and line up patients for cohor

Seel, do trials try and line up patients for cohorts as early as possible?  Would DF be recruiting now trying to fill all the slots for the expected 31 positions still open for the remainder of the PH1 trial or do they start recruiting for a cohort only when the previous cohort has been deemed safe?  I'd like to think they have cohorts 4 and 5 already lined up so when 3 gets finished they can go right to 4 or, if someone has a bad reaction, they have the 6 (or at least 4 new and the two remaining good cases from 3) ready to go again right away.