Figure 12-2. Photographs of mouse eggs (x600). A. Two-cell egg from oviduct 24 hours after copulation. Large second polar body and disintegrating first (on opposite side). B. Seven-cell egg from oviduct 48 hours after copulation. Note one cell on left larger than the rest. Division of this cell would give the eight-cell stage. ( A from Lewis and Wright, 1935.)

Lewis, W.H., and E.S. Wright. 1935. On the early development of the mouse egg. Carnegie Inst. Wash. Pub. No. 459: 133-144.