After repeated trials with dilute solutions of methylene blue upon larvæ and embryos of a number of the smaller decapod Crustacea, a favourable object for the study of the nervous system was at last found in the embryo of the common lobster. The embryos of this animal are specially advantageous on account of their large size, which enables them to be readily manipulated without much damage being done to their tissues, and also on account of the large size of the individual nerve elements.