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[1] [2] Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form. [3]
Coregonus is a diverse genus of fish in the salmon family ().The Coregonus species are known as whitefishes.The genus contains at least 68 described extant taxa, but the true number of species is a matter of debate.
Gastrulation in frogs commences in the marginal zone— the region encircling the blastula's equator where the animal and vegetal hemispheres meet—differing from sea urchins where it begins in the most vegetal part. In contrast to the vegetal blastomeres, the endodermal cells in the marginal zone of frogs are smaller and contain less yolk. [2]
The blastocoel further expands and the inner cell mass becomes positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. The axis formed by the inner cell mass and the blastocoel is the first axis of symmetry of mammalian embryo and determines its attachment point to the uterus.
Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula. [ 19 ] [ 20 ] [ 21 ] The blastocyst is similar in structure to the blastula but their cells have different fates.
There is much variation among the European whitefish forms, but in general they have a tapered body, a slightly protruding upper jaw and a fleshy dorsal fin that is typical of the salmon family. The snout is short and tapered, a fact that distinguishes this species from the two other North European Coregonus species, vendace ( Coregonus albula ...
Even at this depth, there is an underwater current, with a velocity of 0.5–1 m/s (1.6–3.3 ft/s), that disperses some of the deposited eggs downstream in the tributary system. Fertilized eggs are 3.1–3.25 mm (0.12–0.13 in) in diameter and are held in spaces between rocks as well as in the ice crystals of the slushy, tributary floor. [ 8 ]
The broad whitefish is a herring-shaped fish with a more compressed body and convex head than other whitefishes. It is iridescent, with a dark olive-brown back, silvery grey sides, and a whitish bottom. [2] [3] Features that distinguish it from other species include a mild overbite and 18–25 short gill rakers.