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1. Distinct from any cell of the mother or father
2. Human
3. A complete or whole organism, though immature
While looking down upon the top of the embryonic disk, we can see that it is shaped like a serving platter- slightly oval.
What will happen is over the next week, the embryo will elongate (one side becoming the head and the other, the rump)
The embryo develops further
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Much of the morphological change concerns the formation of extraembryonic structures, such as the amniotic cavity and amnion, the primary and secondary yolk sacs, and the connecting stalk, and the chorionic stalk
NO! The human embryo, zygote, or fetus can be nothing other than a human being!
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The First Week: Cleavage and Implantation
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Much of the second week after fertilization is still concerned with implantation, which is not completed until day ten to twelve.
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Gastrulation begins with the migration of cells to the center and back of the embryonic disc.
The newly formed zygote is not yet in the uterus yet. He must make a journey there.
Even though he does not grow in size, he begins his first processes of biological development.
George, Robert P., and Christopher Tollefsen. Embryo: A Defense of Human Life. New York: Doubleday, 2008. Print.
Schmidt, Eric. "Google Images." Google Images. Google, 4 Sept. 1998. Web. 05 Sept. 2015.
Implantation makes it possible for the embryo to receive oxygen and nutrition from the mother.
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The various structures necessary for providing the embryo with a suitable environment and adequate nutrition are in place at such an early time.
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However, prior to the primitive streak, cells are sufficiently uncommitted to specialized roles so that when their processes are disrupted, twinning is possible.
These are also referred to as "cleavages". The resulting cells are smaller and enclosed within the "zona pellucida"
Claire Kotwicki, Brooke Burke, Sammie Escamilla, Aidan Gelbach
Dr. Gildersleeve
Beginning of Life 4A
September 25, 2015
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This is why until the primitive streak happens, it is only possible for the embryo as a whole to divide, with each new embryonic whole capable of developing toward the mature stage of a human being.
The first cleavage is from one cell to two. The following cleavages are asynchronous (not occurring at the same time). These cells of the embryo are called blastomeres.
By three days after conception, the embryo contains around sixteen cells and look something like a blackberry.
At about eight to ten cell stage, the blastomeres undergo a process of "compaction". The blastomeres change their shape and become tightly aligned.
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At this stage, the morula has an inner group of cells called the inner cell layer and an outer cell layer.
The cells of the trophoblast send specific biomolecular signals to the cells of the inner cell mass. This promotes their further development.
Two changes are particularly important.
William Larson wrote, “Male and female sex cells unite at fertilization to initiate the embryonic development of a new individual.”
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The first part, a thin outer layer called the trophoblast, is the progenitor of the placenta.
The whole embryo at this point is referred to as a blastocyst
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The second part, the inner cell mass called the embryoblast, gives rise to the early embryo proper.
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Going back to the primitive streak, it extends toward the middle of the embryo until about eighteen days, at which time a different line of cells, the notochord, emerges.
At four or five days, the morula-stage embryo enters the uterus!
This disc gives rise to the germ layers that form all tissues and organs of the embryo
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The first change is the division of the inner cell mass into a two-layered embryonic disc (in the third week, a third layer is generated).
The distinction between the inner and outer becomes more pronounced.
A fluid-filled space called the blastocyst cavity seperates the embryo into two parts.
The second change is that in one of the two layers, they hypoblast (the other is the epiblast), the prechordal plate develops which indicates the future cranial region of the embryo and the future site of the mouth. The prechordalplate is also an important organizer of the head region.
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By the end of the third week, the embryo has gotten himself ready to enter the next four-week phase of intense structural development, the period of morphogenesis.
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By 10 to 12 days after fertilization the embryo is completely embedded in the endometrium. It takes in nourishment from surrounding maternal issues.
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Over the next two days, the zona pellucida degenerates, and the embryo eventually pierces a hole in it, from which he exits the zona membrane.
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This is called hatching. The embryonic human then grows rapidly. Six days after fertilization, the blastocyst attaches to the uterine lining.
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