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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]
After the somatic cell transfers, the cytoplasmic factors affect the nucleus to become a zygote. The blastocyst stage is developed by the egg to help create embryonic stem cells from the inner cell mass of the blastocyst. [3] The first mammal to be developed by this technique was Dolly the sheep, in 1996. [4]
Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo [2] after passing through an organizational checkpoint during mid-embryogenesis. [3]
During this stage, the zygote divides in a process called cleavage. A blastocyst is then formed and implants in the uterus. Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.
Cavitation is the formation of the blastocoel, a fluid-filled cavity that defines the blastula, or in mammals the blastocyst. [1] After fertilization , cell division of the zygote occurs which results in the formation of a solid ball of cells ( blastomeres ) called the morula .
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.
The zygote will develop into a male if the egg is fertilized by a sperm that carries a Y chromosome, or a female if the sperm carries an X chromosome. [5] The Y chromosome contains a gene, SRY , which will switch on androgen production at a later stage leading to the development of a male body type.
During these cellular divisions, the zygote remains the same size, but the number of cells increase. The morula enters the uterus after three or four days—during which a cavity, called the blastocoel, is formed to produce the blastocyst. [4] Once the blastocyst is formed, it undergoes implantation into the endometrium. [4]