The tinctorial reactions to blood stains of the cells of the perivisceral fluid of the sea-urchin Arbacia punctulata (Lam''k.) at Woods Hole, Mass., are as follows: (a) leucocytes, non-granular cytoplasm without specific reaction; (b) spherules of "amebocytes with colorless spherules," amphophilic to basophilic; (c) spherules of "amebocytes with red spherules," strongly acidophilic; (d) spherules of "amebocytes with yellow spherules," faintly acidophilic. "Amebocytes with colorless spherules" are inferred to be genetically related to the leucocytes because: (a) leucocytes ingest food droplets in the intestinal wall; (b) the food droplets apparently coalcsce to form spherules of the size and nature of the colorless spherules; (c) "amebocytes with colorless spherules" are always present in the region of food ingestion; (d) "amebocytes with red spherules" are rarely present here. "Amebocytes with red spherules" are assumed to be the end products of intracellular digestion of colorless spherules, because of the small size of the red spherules, the tinctorial reaction of the red spherules, the great abundance of these cells in the perivisceral fluid, their emigration from the body, and their degeneration in the test. The wall of the spherule is stained black by osmic acid, but the content is not stained. Hence it is inferred that the wall is composed of a lecithin and the content is an albuminoid.