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.

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.

Vulnerability During Prenatal Development

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

University of Chicago

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Prenatal development encompasses the orderly and delicate sequence of growth from a single-cell structure to birth. This crucial period of human development is typically divided into three main stages: germinal, embryonic, and fetal.

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

Stages of Prenatal Development 

The documentary 'Life's Greatest Miracle,' produced by Nova and PBS, provides a detailed visual account of the stages of prenatal development and the process of birth.

Life's Greatest Miracle (Video Resource)

During each prenatal stage, there are many factors (e.g., genetic and environmental) that can affect one's development. These may include:
- Prenatal care
- Teratogens

Prenatal Influences

A pregnant individual is exposed to a substance known to interfere with the initial formation of major organs. Explain why this exposure would be significantly more likely to cause severe structural birth defects if it occurs during the first eight weeks of gestation compared to an exposure that occurs during the final three months of pregnancy.

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