The Xenopus egg is 1.2-1.4 mm in diameter, and consists of a darkly pigmented "animal hemisphere" and a lighter yolky "vegetal hemisphere." When laid, the eggs are oriented randomly with respect to gravity, and held in position by a transparent vitelline membrane inside a jelly coat (Fig. 1), but after fertilization, granules located just below the surface of the egg fuse with the plasma membrane and release their contents into the space between the vitelline membrane and the egg. This material provides some lubrication, allowing the egg to rotate such that the less dense animal hemisphere is uppermost. This rotation usually occurs within 20 min of fertilization.
Xenopus is monospermic, and the successful sperm enters the egg in the animal hemisphere. The site of sperm entry (the "sperm entry point," or SEP) is often visible as a small aggregation of pigment. The position of sperm entry defines the future dorsal-ventral axis of the embryo: the future "dorsal" side of embryo forms from the side of the egg opposite the SEP. (Dorsal is in quotation marks in the previous sentence, because the dorso-ventral axis of the egg is not directly translated into the dorso-ventral axis of the tadpole; see below.) The SEP defines the dorso-ventral axis of the embryo by determining the direction of rotation of a cortical layer of cytoplasm, just beneath the plasma membrane. This rotation, of about 30°, is driven by a transiently aligned microtubule array in the vegetal hemisphere of the egg and begins about 40 min after fertilization. Through mechanisms that are still completely unclear, the rotation establishes a signaling center, often referred to as the "Nieuwkoop Center," which directs the development of the dorso-anterior region of the embryo (see below).
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