To experimentally test the suggestion made in the preceding paper that the microtubules are involved in cell shape development during the formation and differentiation of the primary mesenchyme, we applied to the embryos two types of agents which affect cytoplasmic microtubules: (a) colchicine and hydrostatic pressure, which cause the microtubules to disassemble, and (b) D2O, which tends to stabilize them. When the first type of agent is applied to sea urchin gastrulae, the development of the primary mesenchyme ceases, the microtubules disappear, and the cells tend to spherulate. With D2O development also ceases, but the tubules appear "frozen," and the cell asymmetries persist unaltered. These agents appear to block development by primarily interfering with the sequential disassembly and/or reassembly of microtubules into new patterns. The microtubules, therefore, appear to be influential in the development of cell form. On the other hand through a careful analysis of the action of these agents and others on both intra- and extracellular factors, we concluded that the microtubules do rather little for the maintenance of cell shape in differentiated tissues.

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