Driesch’s 'pressure plate' experiment demonstrated:
Hans Driesch isolated individual blastomeres of two-cell sea urchin embryos and compressed embryos between glass plates to alter cleavage planes, forming pressure plate experiments. Despite displaced positions, each isolated blastomere and each pressure-altered embryo developed into complete although smaller pluteus larva, not half embryo. This regulative behavior demonstrated conditional specification where cell fate depends on interactions with neighbors and position, not autonomous inheritance. Result contrasted with Wilhelm Roux's mosaic interpretation, establishing principle of embryonic regulation and developmental plasticity fundamental to deuterostome development and induction models used today.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Driesch pressure plate conditional specification regulative development.