Ans. Embryonic development occurs in distinct phases, starting from a single fertilized cell (zygote) to a fully formed organism. The major phases include:
1. Gametogenesis
(Formation of Gametes)
The process by which sperm
(spermatogenesis) and egg cells (oogenesis) are formed through meiosis.
2. Fertilization
Fusion of sperm and egg to
form a zygote.
Restores the diploid
chromosome number and activates embryonic development.
3. Cleavage
Rapid mitotic cell
divisions occur, forming a morula (solid ball of cells).
No overall growth in size,
but the number of cells increases.
4. Blastulation
Formation of a blastocyst
(hollow ball of cells).
The inner cell mass (ICM)
develops into the embryo, while the trophoblast contributes to placenta
formation.
5. Gastrulation
Cells rearrange into three
germ layers:
Ectoderm → Forms skin,
brain, and nervous system.
Mesoderm → Forms muscles,
bones, and circulatory system.
Endoderm → Forms digestive
and respiratory organs.
6. Neurulation
(Formation of Nervous System)
The neural tube forms,
which later develops into the brain and spinal cord.
7. Organogenesis
(Formation of Organs)
Germ layers give rise to
specific organs and tissues.
Major organs start to
develop by the end of the embryonic period (8 weeks in humans).
8. Growth and
Differentiation
The fetus grows in size,
and cells differentiate into specialized tissues and Organs.