An embryo is the designation for a multi-cellular organism after it has successfully implanted in the lining of the uterus. Following implantation, the embryo's basic structures begin to differentiate and develop into areas that will form the head, chest, and abdomen.

Embryo

During the embryonic stage, a critical structure called the neural tube forms along the dorsal side (back) of the embryo. This tube is the foundational structure of the central nervous system, as it subsequently develops into the spinal cord and brain.

Neural Tube Formation in the Embryo

The placenta begins to form after the zygote attaches to the uterine wall, a key event of the early embryonic stage. This organ serves as a crucial interface between the pregnant person and the developing fetus, facilitating the transfer of nourishment and oxygen. It is also the pathway through which most substances ingested by the pregnant person, such as food, liquids, and medications, reach the fetus.

Placenta Formation and Function

A developing human is particularly vulnerable to environmental influences during the period that begins about two weeks after conception and lasts for approximately six weeks. Analyze why this specific timeframe is considered a period of heightened risk, focusing on the key developmental processes that occur.

Vulnerability in Early Development

The embryonic stage, lasting for about six weeks after the zygote implants in the uterus, is the second phase of prenatal development. During this critical period, the embryo's body and organ structures begin to form. A key development is the formation of the neural tube, which serves as the precursor to the brain and spinal cord.

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There are three stages of prenatal development:
1. Germinal Stage
2. Embryonic Stage
3. Fetal Stage 

Stages of Prenatal Development 

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Embryonic Stage

The fetal stage is the third and final phase of prenatal development, commencing at the ninth week and extending until birth. This period is primarily characterized by the rapid growth of the fetus's body, brain, and organs as they mature in preparation for life outside the womb.

Fetal Stage

The germinal stage, lasting from conception through the first two weeks of prenatal development, is characterized by conception and rapid cell division through mitosis. During this fragile period, the mass of cells has not yet attached to the uterine lining, and fewer than half of all zygotes survive this initial stage.

Germinal Stage

An expectant mother learns that she was unknowingly exposed to a harmful substance approximately four weeks after conception. This substance is known to cause severe defects in the formation of the heart, limbs, and central nervous system. Based on the timing of the exposure and the types of defects mentioned, identify the stage of prenatal development during which the developing organism was most vulnerable. Justify your answer by explaining the key developmental processes occurring during this stage.

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