Diagram indicating the pigmentation differences and nomenclature of cells from the one-cell to the 32-cell stage. The top row represents a left (L) side view. The bottom row represents an animal pole view. The point of sperm entry is indicated at the one-cell stage. an, animal pole; veg, vegetal pole; CS, cell stage; D, dorsal; V, ventral; L, left; R, right. Nomenclature is that of Jacobson (Hirose and Jacobson 1979; Jacobson and Hirose 1981). Tiers on the 32-cell embryos refer to the Nakamura and Kishiyama (1971) nomenclature.