Embryonic Development in Mammals
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The Zygote
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As the zygote matures, it goes through a series of rapid divisions known as cleavage. The zygote does not put on any additional mass nor does it increase or decrease in size during this period. There are two types of cleavage that occurs, depending on the embryo's source of nourishment during its development. For those embryos that receive the majority of its nourishment through a placenta and umbilical cord, the cleavage is known as holoblastic. For those embryos that will receive the majority of their nourishment through an egg yolk, the process is known as meroblastic. Holoblastic cleavage occurs in mammal embryonic development.
After cleavage, the zygote will now be known as a blastula. Mammals will develop a blastocyst in addition to a blastula, and, even though they are structurally similar, they are not the same structure. Cells will begin to migrate into the center of the blastula and begin to form primitive germ layers, transforming the blastula into a gastrula.
Advanced Development
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It is at this point that cellular specialization begins to occur. Somites form and begin to block off and form the skeletal muscles, vertebrae and dermis for all mammal embryos. After the somites divide to form the basis for the spinal column, organogenesis begins. Organogenesis begins the development of the different organ systems including the nervous, circulatory and digestive.
Eventually, the development of the embryo is complete, with the embryo now being known as a fetus. The remainder of the gestation period is devoted to growing and maturing the fetus, preparing it for birth. The embryo has become viable at this point, though its survival may be limited, as, even though the embryo is now fully formed, the organ systems may not be fully mature.
Every mammal embryo begins with the same process. It is only the DNA and chromosomes that coordinate the differentiation of the cellular structures and the determination of the species. The zygotes and early embryos of mammals are virtually indistinguishable from each other until the organogenesis process begins and the embryo begins to assume its future characteristics.
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