In this article we will discuss about the gastrulation and the formation of primary organ rudiments in fishes.
In fishes the yolk formed in the oocytes during oogenesis initially does not differ substantially from the yolk in amphibian oocytes and eggs. It consists of yolk platelets lying in the cytoplasm and intermingled with other cytoplasmic components (mitochondria and the like).
The cores of the yolk platelets, similar to those of amphibians, may show a crystalline structure, although this structure is lost by the time the eggs are ripe. In the immature egg the surface is formed by yolk-free cytoplasm containing numerous cortical granules.
After fertilization, however, the cytoplasm moves toward the animal pole of the egg, and there it forms a cytoplasmic “cap.” This is particularly the case in bony fishes. Active cytoplasm is withdrawn both from the surface and from the interior of the egg.
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As a result, the yolk platelets are pressed closer together. The process is a gradual one and occurs earlier or later in different fishes. In the more primitive fishes the condensation of the yolk does not go too far; cleavage remains complete, and development follows essentially the pattern found in amphibians.
In the Teleostei, however, the yolk platelets are pressed together in the central parts of the