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E . N . W I L L M E R
FIG. 1. Four-day old culture of thymus tissue of rat in hanging drop of plasma. (Photograph
by Trowell.)
The structure and function of each and every cell in any organ in
situ depends partly on the genetical constitution of the cell and partly
on environmental characteristics imposed on the cell during the course
of embryonic development. It also depends on the exact micro-environ-