Henry Gray (1825–1861). Anatomy of the Human Body. 1918.
FIG. 9
First stages of segmentation of a mammalian ovum. Semidiagrammatic. (From a drawing by Allen Thomson.) z.p. Zona striata. p.gl. Polar bodies. a. Two-cell stage. b. Four-cell stage. c. Eight-cell stage. d, e. Morula stage.
