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In frog embryos, gastrulation initiates at the site identified as the gray crescent, located on the future dorsal side of the embryo, slightly below the equatorial region. This process involves cells migrating inward to form a structure similar to a blastopore .
Gastrulation has been studied in many animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals that develop outside the mother. Model organisms whose gastrulation is understood in the greatest detail include the mollusc, sea urchin, frog, and chicken.
The primitive node (or primitive knot) is the organizer for gastrulation in most amniote embryos. In birds, it is known as Hensen's node, and in amphibians, it is known as the Spemann-Mangold organizer. It is induced by the Nieuwkoop center in amphibians, or by the posterior marginal zone in amniotes including birds.
During gastrulation cells migrate to the interior of the blastula, subsequently forming two (in diploblastic animals) or three (triploblastic) germ layers. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesoderm and endoderm.
The Spemann-Mangold organizer is a group of cells that are responsible for the induction of the neural tissues during development in amphibian embryos.First described in 1924 by Hans Spemann and Hilde Mangold, the introduction of the organizer provided evidence that the fate of cells can be influenced by factors from other cell populations. [1]
Modern fate mapping began in 1929 when Walter Vogt invented a process which involved marking a specific region of a developing embryo using a dyed agar chip and tracking the cells through gastrulation. [3] To achieve this experiment, Vogt allowed dye and agar to dry on a microscope plate, and placed small pieces onto specific embryo locations.
The primitive streak is a structure that forms in the early embryo in amniotes. [1] In amphibians, the equivalent structure is the blastopore. [2] During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the broad end towards the anterior, and the narrower region projected to the posterior.
In the frog Xenopus laevis, the animal pole is heavily pigmented while the vegetal pole remains unpigmented. [4] A pigment pattern provides the oocyte with features of a radially symmetrical body with a distinct polarity. The animal hemisphere is dark brown, and the vegetal hemisphere is only weakly pigmented.