Question

briefly explain how the microscopic cell that begins human life is formed.

list and briefly describe two important structures that form outside the embryo during the early stages of development.
1.
2.

list the stages of embryonic and fetal development and identify one major change that takes place during each stage.

stage of development

major changes

Explanation:

1. The microscopic cell that begins human life is a zygote, formed by the fusion of a sperm and an egg during fertilization.
2. Two important extra - embryonic structures in early development:

• Yolk sac: It provides early nourishment to the embryo and is involved in early blood cell formation.
• Amnion: It encloses the embryo in a fluid - filled sac, providing protection against mechanical shock.

1. Stages of embryonic and fetal development and major changes:

• Germinal stage (first 2 weeks): The zygote undergoes rapid cell division (cleavage) and forms a blastocyst, which implants in the uterine wall.
• Embryonic stage (3 - 8 weeks): Organogenesis occurs. Major organs and body systems begin to form, such as the heart starting to beat around 3 weeks.
• Fetal stage (9 weeks - birth): Growth and maturation of organs. For example, in the third trimester, the lungs mature to prepare for breathing air after birth.

Answer:

1. The microscopic cell that begins human life is a zygote formed by sperm - egg fusion during fertilization.

2.

• Yolk sac: Provides early nourishment and is involved in early blood cell formation.
• Amnion: Encloses embryo in a fluid - filled sac for protection.

3.

• Germinal stage (first 2 weeks): Zygote undergoes cleavage, blastocyst forms and implants.
• Embryonic stage (3 - 8 weeks): Organogenesis, heart starts to beat.
• Fetal stage (9 weeks - birth): Organs grow and mature, lungs mature in third trimester.