The number of cells in the gut of a sea urchin embryo was counted to clarify the cellular mechanisms of gut formation during development. It has been determined that in the gut of a normal embryo, the number of cells amounted to at least 49, 100, 181, 175 and 169 at the early gastrula, mid-gastrula, late gastrula, prism and pluteus stage, respectively. The percentage of the number of cells in the gut to the total number of the cells in the whole embryo was the highest at the late gastrula stage.