The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side. 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 amphibians the equivalent structure is the blastopore. Changes in bone, internal organs and skin appear during metamorphosis.The primitive streak is a structure that forms in the early embryo in amniotes. This larva develops further to the premetamorphose stage which will be characterized by: a broad head, two palps at both sides of the mouth, fore and back legs and a long tail laterally flattened and with a web at the dorsal and ventral side. The head, the body and the tail region are clearly visible. In this picture, we look to the dorsal side. The tail bud stage develops to a real larva. Furthermore, the place of the stomodeum (presumptive mouth) and the first external shape of the gills and tail are visible. The rudimentary eyes are clearly distinguishable. This axis becomes clearly visible during further elongation growth of the embryo and the presumptive forebrain, eye vesicles and gill plates are developing.ġ day and 7 hours. The anterior-posterior body axis is appearing now due to the extension of the embryo. In this figure, the transition of neural groove to neural tube is shown.Īfter 24 hours. The neural walls develop at each side of the groove and grow to each other to form the neural tube 24 hours after fertilization. One hour later the cells of this neural plate moves inwards in the median region of the plate to form the neural groove. The ectoderm at the dorsal side of the gastrula thickens about 14 hours after fertilization and forms the neural plate as a presumptive structure of the nervous system. The neurulation process induces the formation of the nervous system. At the end of the gastrulation process, this opening is completely filled with a dense mass of yolk rich cells: the yolk plug.īetween 15 and 24 hours. The circular blastopore is formed by the process of involution and epiboly whereby the dorsal, the ventral and the two lateral blastopore lips grow to each other. The cells at the surface of the dorsal side move inside (invaginate) at the dorsal blastopore lip.ġ2 hours. First, the blastopore becomes visible due to so-called morphogenetic movements (migration of blastomeres). The gastrulation process starts after the blastula stage (not shown) in which the micromeres are sometimes difficult distinguishable as individual cells (see further in figure F).ġ0 hours. The dorsal micromeres are mostly smaller and less pigmented than the ventral macromeres.ħ - 10 hours. Now, at the sixteen cell stage, the cleavage furrows are completed and the macromeres are fully separated. Earlier, at the eight cell stage (not shown), the third cleavage occurred horizontally and just above the equatorial plane, resulting in a difference in largeness of the blastomeres: micromeres at the animal pole and macromeres at the vegetal pole. The cleavage is not always complete due to the higher concentration of yolk at the vegetal pole.Ģ - 3 hours. The four formed blastomeres can differ in largeness and color. The second vertical cleavage is completed at right angles to the first one. The first cleavage plane corresponds to the plane of bilateral symmetry in the developing embryo.Ģ hours. The first cleavage occurs in a vertical plane and the cleavage furrow begins at the animal pole and slowly extends down into the vegetal region. The position of the grey crescent determines the orientation of the bilateral symmetry in the egg and the developing embryo. After penetration of the sperm, a grey crescent is formed at the opposite side of the point of sperm entry due to the rotation of the cortical cytoplasm. The sperm penetrates the animal hemisphere mostly under an angle of about 40° with respect to the apical pole of the oocyte (not visible here). One cell stage, shortly after fertilization. On this page a number of characteristic stages of the embryonic development in fixed in toto embryos of the tadpole Xenopus laevis can be seen.Ġ - 1 hour.
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