The frog egg is a huge cell; its volume is over 1.6 million times larger than a normal frog cell. During embryonic development, the egg will be converted into a tadpole containing millions of cells but containing the same amount of organic matter. * The upper hemisphere of the egg — the animal pole — is dark. * The lower hemisphere — the vegetal pole — is light. * When deposited in the water and ready for fertilization, the haploid egg is at metaphase of meiosis II
Entrance of the sperm initiates a sequence of events:
* Meiosis II is completed.
* The cytoplasm of the egg rotates about 30 degrees relative to the poles. * In some amphibians (including Xenopus), this is revealed by the appearance of a light-colored band, the gray crescent. * The gray crescent forms opposite the point where the sperm entered. * It foretells the future pattern of the animal: its dorsal (D) and ventral (V) surfaces; its anterior (A) and posterior (P); its left and right sides. * The haploid sperm and egg nuclei fuse to form the diploid zygote nucleus. Cleavage
The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells. The first cleavage occurs shortly after the zygote nucleus forms. * A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. * This divides the egg into two halves forming the 2-cell stage The second cleavage forms the 4-cell stage. The cleavage furrow again runs through the poles but at right angles to the first furrow. The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal pole. It produces the 8-cell stage. The next few cleavages also proceed in synchrony, producing a 16-cell and then a 32-cell embryo. However, as cleavage continues,...