= Solution
If the <biological cells> are initially interchangeable, randomly allocate them to five groups of ten for the days. Within each day, independently choose a <uniform distribution on a finite set> over assignments having two occurrences of each dose level to the ten <experimental units>. One implementation is a random <permutation> of a list containing two labels of each level; the duplicate labels give every admissible assignment the same number of underlying permutations.
Randomize the processing order within each day as well, while pairing each preparation with its immediate measurement. \b[Randomize dose within day, not merely the names of the days.] This <restricted randomization> preserves the planned balance while protecting <treatment contrasts> against systematic order effects. Conceal the dose labels from the assessor where practical.
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