![]() ![]() During involution, these inner cells move toward the dorsal lip and then fold over themselves and progress toward the animal pole (the top of the embryo). As these cells divide and extend to envelope the inside of the developing frog, the marginal zone cells involute (or fold inward) at the dorsal lip of the blastopore. Once the blastopore has formed, the cells of the animal hemisphere undergo epiboly (flattening of the outer layer of cells and radial intercalation of the layer of cells beneath them) and move toward the blastopore. The lip of the blastopore nearer to the animal pole is termed the dorsal lip of the blastopore. Here, cells undergo apical constriction to become “bottle” cells (tapered cells resembling narrow Erlenmeyer flasks) since most of the cytoplasm in each of these cells is made to migrate toward the center of the embryo, the part of the cell in contact with the groove to become much narrower. The blastopore is a groove in the side of the embryo that results from the invagination (the formation of a groove) of a small group of future endodermal cells, and forms right below the equator of the embryo, in the marginal zone. The prospective ventral side of the embryo is on the side of the sperm’s entry, while the prospective dorsal side, the side at which the blastopore forms, is opposite the sperm’s point of entry. While the animal-vegetal gradient is determined in the egg prior to fertilization, the point of entry of the sperm lends the frog its dorsal-ventral (back-and-front) axis. ![]() The blastocoel occupies most of the inside of the animal hemisphere, since the cells in the vegetal pole bear the yolk of the embryo and therefore occupy more of the inner space in that region. The equator between the two hemispheres is called the marginal zone. Cellular division occurs much more rapidly near the animal (active) pole of the frog embryo than near the yolkier vegetal (sedentary) pole the yolk provides the embryo with nutrition, but slows down cell division around it. The hemispheres of the blastula correspond to the names of their respective poles. If the blastula were compared to a globe of the world, the North Pole would correspond to the animal pole of the embryo, and the South Pole to the embryo’s vegetal pole. The raw material for gastrulation is the blastula, a hollow sphere of cells the space inside of the blastula is called the blastocoel. The following is a detailed explanation of gastrulation in Xenopus while gastrulation varies across species, studies in Xenopus have shed considerable light on the process in general. The size and structure of Xenopus laevis (African clawed frogs) embryos have made the species into a model organism for early developmental study. Since the early twentieth century, experimental embryologists like Hans Spemann and Wilhelm Roux have extensively studied gastrulation in amphibian embryos in an attempt to learn more about how establishment of different regions in the body is determined. The gastrula, the product of gastrulation, was named by Ernst Haeckel in the mid-1870s the name comes from Latin, where gastēr means stomach, and indeed the gut (archenteron) is one of the most distinctive features of the gastrula. In diploblastic organisms like cnidaria or ctenophora, only the endoderm and the ectoderm form in triploblastic organisms (most other complex metazoans), triploblastic gastrulation produces all three germ layers. The process of gastrulation can be either diploblastic or triploblastic. The process of gastrulation allows for the formation of the germ layers in metazoan embryos, and is generally achieved through a series of complex and coordinated cellular movements.
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