Beginning of Segmentation

Abstract

Within 24 hours the first cleavage division is completed. The eggs are near the exit of the ampulla, between the first and the second loop of the oviduct (Fig. 9). This second loop is said to exhibit peristaltic contractions, which aid ova transport [5]. The first cleavage yields two cells of about equal size, with finely granulated cytoplasm. Their large spherical nuclei contain 4–5 nucleoli that are surrounded by a small border of chromatin. The egg and zona pellucida shrink considerably after Bouins fixation. The overall diameter is 48 to 64 microns, including the zona. After fixation with OsO₄ there is much less shrinkage and the zona appears in sections as a distinct thick ring (Figs. 10 and 11), with an overall diameter of 83 microns. In the fresh, unfixed state, eggs of this stage are 80–100 microns in diameter.