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In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blasto-disc, on top of the yolk. Discoidal cleavage is commonly found in monotremes, birds, reptiles, and fish that have telolecithal egg cells (egg cells with the yolk concentrated at one end). The layer of cells that have ...
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus .
Because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. [8] [9] In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first.
The division of blastomeres from the zygote allows a single fertile cell to continue to cleave and differentiate until a blastocyst forms. The differentiation of the blastomere allows for the development of two distinct cell populations: the inner cell mass, which becomes the precursor to the embryo, and the trophectoderm, which becomes the precursor to the placenta.
Summary of the main patterns of yolk accumulation and cleavage in animal embryology (after [6] and [7]). I. Holoblastic (complete) cleavage II. Meroblastic (incomplete) cleavage A. Isolecithal (sparse, evenly distributed yolk) 1. Radial cleavage (echinoderms, hemichordates, amphioxus) 2. Spiral cleavage (annelids, most mollusks, flatworms) 3.
The Department of Government Efficiency (DOGE), which is led by tech billionaire Elon Musk, is taking aim at the U.S. Agency for International Development (USAID) and, according to senior ...
As cleavage continues, the cavity expands to become the developed blastocoel. The blastocoel is a crucial component of amphibian embryo development. It permits cell migration during gastrulation and prevents the cells beneath the blastocoel from interacting prematurely with the cells above the blastocoel.
In Kennesaw, Georgia, it's against the law not to own a gun.. The city law, dating back to the 1980s, makes it a requirement for residents to own guns and ammo. Kennesaw's gun law states: "In ...