Amphibians accomplish fertilization of their eggs in a variety of ways. External fertilization, employed by most frogs and toads, involves a male holding a female in a pose called amplexus. In amplexus, the male releases sperm over the female’s eggs as they are laid. Less risky is the method employed by many salamanders whereby the male deposits a packet of sperm called a spermatophore onto the ground. The female then pulls it into her cloaca where fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization just like reptiles, birds and mammals. The male deposits sperm directly into the female’s cloaca via an intromittent organ.
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.
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