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Embryonic Development- A Brief Look From Conception to Viability

Updated on September 27, 2014

The Zygote

The initial stage of the Zygote period is defined by that time between fertilization and attachment to the uterus. During this period cell division begins and a blastocyst is formed. The inner cells will go on to form an Embryo and the outer cells will form the amnion.

Next, once implantation occurs, the blastocyst becomes embedded in the uterine lining while the amnion encases fluid meant to provide temperature regulation and cushioning. The yolk sac begins to produce blood cells.

During the last part of the Zygote period the chorion forms around the amnion and villi insert themselves into the uterus and begin the formation of the placenta. The umbilical cord will provide a connection between the placenta and the burgeoning Embryo. This period reaches its end about two weeks after fertilization.

The Embryo

The Embryo differentiates into three layers of cells in the third week: The ectoderm, the mesoderm, and the endoderm.

The second significant phase for the Embryo is the formation of the neural tube and the development of massive amounts of neurons that occurs within. At the same time, cells differentiate into muscles, bones, and specific organ systems including the heart.

Lastly, during the second month, physiognomy and limb development begins as does continued organ growth including the development of the renal system rendering the yolk sac unnecessary.

The Fetus

By the third month the Fetus period begins and the nervous system can send signals to muscles that have connected to bone for purposeful movement. Autonomic expansion and contraction of the lungs also begins.

Next during this period, between four and five months, or the second trimester, the vernix and the lanugo form to protect the fetal skin. Glial cells continue to be produced.

Finally, after seven months viability is reached, although the bronchioli are not yet functional. The fetus spends more time awake as the cerebral cortex increases in size and connects to other areas of the brain. A layer of fat is added, antibodies are acquired from the mother, and the fetus assumes an inverted position in preparation for birth.


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