Fig. 7. Effect of cell ablation before third cleavage on 1d division pattern. (A) Normal division chronology of first-quartet micromeres. Minutes after first cleavage are indicated on vertical axis; horizontal gray lines indicate the ages of embryos shown in B, F and G (bfg) and in C, D and E (cde). (B-G) Camera lucida drawings of nuclei in whole and partial embryos, shown at the same scale; first-quartet micromere derivatives are shaded. (B) Normal embryo, 500 minutes after first cleavage. The 1b¹² nucleus is in prometaphase; the small 1d¹² nucleus is in interphase. (C) Nuclei in a representative D quarter embryo isolated in the mid four-cell stage and fixed at 375 minutes after first cleavage; 1d¹ is precociously in metaphase. (D) Another D quarter isolated in the mid four-cell stage, and fixed at the same stage as the specimen in C; both 1d¹ and 1d² are in metaphase. (E) Nuclei in a D quarter embryo isolated in the early eight-cell stage and fixed at 375 minutes after first cleavage. Interphase 1d¹ and 1d² nuclei exhibit their normal size difference. (F) Nuclei in a BD half-embryo isolated in the mid four-cell stage and fixed at 500 minutes after first cleavage, showing two abnormal pairs of nuclei apparently derived from 1d. The 1b lineage has a normal appearance, including one small nucleus (1b²) and two larger ones (1b¹¹ and 1b¹²); the 1b¹² nucleus is in metaphase. (G) Nuclei in a BD half-embryo isolated in the early eight-cell stage and fixed at the same stage as the embryo in F. The 1d cell has produced the normal complement of three nuclei. Uncharacteristically, the two daughters of 1b¹ are dividing synchronously. In both F and G, the 3b nucleus has entered prophase earlier than normal, probably as a result of abnormal exposure to 3D signaling. 4d is in metaphase; 4D is not shown.