This includes the zygote, cleavage, and blastula periods (32). Within 10 min of the egg being fertilized, streaming cytoplasmic movements cause a cap of trulation begins at 50% epiboly (5.25 h) by which time the blastomeres have moved down over the yolk cell to cover half its extent (arrows). (D) Animal pole view of the shield stage (6 h) at which the position of axial involution is evident (arrow). (E) At 70% epiboly, the edge of blastomeres (small arrows) has spread further round the yolk cell. The anterior extent of the axially involuting hypoblast can be seen approaching the animal pole (large arrow). (F) The bud stage (10 h) is reached at the end of gastru-lation. The yolk cell is completely covered by cells and the anterior extent of the axial hypoblast can be seen as the polster (arrow). (G) The eighth somite stage (13 h). The arrow marks the eye. (H) 22 somite stage (19 h). The arrow marks the otic vesicle. (I) The prim 20 stage (33 h). Pigmentation is now clear as melanocytes spread over the body and the retinal pigment epithelium of the eye differentiates. Scale bar = 250
clear cytoplasm to emerge at the animal pole of the otherwise yolky cell. After 45 min, the first cleavage occurs incompletely separating the clear cytoplasmic region (Fig. 2A). Subsequent cleavages are more rapid, occurring at 15-min intervals, generating the 64-cell stage by 2 h. The cells adjacent to the yolk cell communicate by intracellular bridges. The blastula stage (Fig. 2B) begins with the generation of 128 cells. Divisions initially remain largely synchronous, but this synchrony begins to be lost at the time of the midblastula transition (34), which begins at the 10th cell cycle (512-cell stage). Other key events that occur during the blastula stage are the formation of the yolk syncytial layer (YSL) and the beginning of the process of epiboly. The YSL is formed as the marginal blastomeres sink into the yolk cell distributing their cytoplasm and nuclei as a layer across its superior edge. Epiboly is the spreading of the blastodisc and the YSL over the surface of the yolk cell. By the end of gastrulation, the yolk cell is completely engulfed. In addition to the YSL and the deep cells (DEL) of the blastodisc, during the blastula stage, the single outer layer of cells flattens to form the enveloping layer (EVL).
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