Pregnancy and Embryonic Development
Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm, and in between a mesoderm soon appears. These three layers give rise to all tissues (organs) in adults.
After implantation, the chorionic villi (finger-like projections) get surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between the developing embryo (fetus) and maternal body called the placenta that is connected to the embryo through an umbilical cord and helps in the transport of substances to and from the embryo. The functions of the placenta are;
- The placenta facilitates the supply of oxygen and nutrients to the embryo.
- It helps in the removal of carbon dioxide and excretory/waste materials produced by the embryo.
- Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
The duration between fertilization and parturition (9 months) is called gestation. During gestation following stages of development are seen;
- 1st month: Heart is formed.
- 2nd month: Limbs and digits develop.
- 3rd month (1st trimester): Limbs and external genital organs well developed.
- 5th month: Shows movements and hair on the head appears.
- 6th month (2nd trimester): The body is covered with fine hair, eyelids separate, and eyelashes are formed.
- 9th month (3rd trimester): The fetus is fully developed and is ready for delivery.
NCERT Notes on Human Reproduction Class 12 Chapter 2
NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic information from each parent, resulting in the formation of a zygote.