1. Newly fertilized eggs and 6-hour blastulae were exposed to sea water solutions of zinc chloride (M/10,000-M/320,000) for varying intervals of time.2. It was found that high concentrations inhibited cleavage while low concentrations caused marked changes in the developmental pattern without much effect in cleavage. The continuous exposure of newly fertilized eggs to strong solutions following fertilization resulted in radial, poorly-differentiated larvae but as the strength of the solution decreased the larvae became elongated in the polar direction and differentiated basal ciliated bands. In the weaker solutions, plutei with large ventral areas differentiated. In M/10,000, exogastrulation occurred in 10-25 per cent of the larvae while the differentiation of skeleton was prevented in all concentrations above M/80,000.3. Continuous treatment after cleavage gave larvae that were but slightly different from those treated continuously from fertilization. The chief differences were that fewer exogastrulae de...