The mesoderm is formed in a ring of cells in the marginal zone, located between the ectoderm and endoderm. The ectoderm, or animal cap, forms the roof of the blastocoel. By the late blastula stage (9 h of development), the three germ layers become defined. As the embryo rapidly divides into smaller and smaller cells, without intervening growth (cleavage), a cavity called the blastocoel is formed, which defines the blastula stage. One hour after fertilization, an unpigmented dorsal crescent is formed in the fertilized egg opposite the sperm entry point. The ovarian oocyte is radially symmetrical and is divided into an animal and a vegetal domain. This dorsal-ventral difference can be traced back to the fertilization stage, with the dorsal side always forming opposite to the sperm entry point. In a complementary experiment, the dorsal lip transplanted to the ventral side of a host embryo induces the formation of a twinned embryo that contains axial structures and organs 2. When the gastrula embryo is bisected by ligature with a hair loop into dorsal and ventral fragments, the half that contains the dorsal blastopore lip develops into an entire embryo, whereas the ventral half remains as a ‘belly piece’ ( Bauchstück) that is devoid of all axial organs 1. At the gastrula stage, the dorsal side of the embryo can be recognized by the presence of the dorsal blastopore lip ( FIG. The dorsal side of the amphibian embryo contains the information for the differentiation of many different cell types. With its rapid embryonic development, large egg size (1-2 mm in diameter) and high numbers of embryos (1,500 per female), Xenopus provides a favourable model system for the study of vertebrate development, and it has been used extensively to probe the events in early embryogenesis.
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