Question
Describe the stages of embryonic development in humans from fertilisation to the formation of the foetus. Name the key developmental stages in order.
Solution — Step by Step
Fertilisation occurs in the fallopian tube (ampullary-isthmic junction), typically within 24 hours of ovulation.
A sperm cell fuses with the secondary oocyte. The oocyte completes its second meiotic division (it was previously arrested at metaphase II). The resulting cell is the zygote — a single diploid cell (46 chromosomes) containing genetic material from both parents.
The zygote is totipotent — it has the potential to form any cell type in the body.
The zygote undergoes a series of rapid mitotic divisions called cleavage. These divisions happen without growth — the cells (blastomeres) get smaller with each division.
Key stages:
- 2-cell stage (≈30 hours after fertilisation)
- 4-cell stage
- 8-cell stage
- Morula (≈72–96 hours) — a solid ball of 16–32 blastomeres, still enclosed in the zona pellucida
The morula travels down the fallopian tube towards the uterus during this time, propelled by ciliary action and smooth muscle contractions.
The morula develops a fluid-filled cavity (blastocoel), transforming into the blastocyst by day 4–5.
The blastocyst has two distinct regions:
- Inner cell mass (ICM) / embryoblast — will become the embryo and some foetal membranes
- Trophoblast — outer layer; will become the placenta and chorion
The blastocyst enters the uterus and implants into the endometrium (uterine lining) around day 6–9. The trophoblast cells invade the endometrium, anchoring the embryo.
This marks the beginning of pregnancy proper.
Around week 2–3, the inner cell mass undergoes gastrulation — the process that establishes the three primary germ layers:
- Ectoderm: outer layer → forms nervous system, skin, sense organs
- Mesoderm: middle layer → forms muscles, bones, connective tissue, heart, kidneys
- Endoderm: inner layer → forms lining of gut, respiratory tract, liver, pancreas
The embryo becomes trilaminar (three-layered). Gastrulation establishes the body axes (head-tail, back-front, left-right).
From week 4 onward, the three germ layers differentiate into organs — this is organogenesis.
Key events:
- Week 4: Heart begins beating, neural tube closes
- Week 5–6: Limb buds appear, facial features begin forming
- Week 8: All major organ systems are present in rudimentary form
- By end of week 8: The embryo is now called a foetus
The transition from embryo to foetus (week 8–9) marks the point when the basic body plan is established and the period of greatest sensitivity to teratogens (substances causing birth defects) has largely passed.
The foetal period (week 9 to birth) is primarily one of growth and maturation of already-formed structures.
Why This Works
The developmental sequence — zygote → cleavage → morula → blastocyst → implantation → gastrulation → organogenesis → foetus — is driven by gene expression programs. Each step is orchestrated by signalling molecules (morphogens) that tell cells where they are and what they should become.
The key principle is cell differentiation: all cells in the embryo have the same DNA, but different genes are active in different cell types. The process of reading different subsets of genes produces the >200 cell types in the human body, all from a single zygote.
Alternative Method
For CBSE/NEET answers, a timeline table format is highly effective:
| Stage | Timing | Key event |
|---|---|---|
| Zygote | Day 1 | Fertilisation, diploid cell |
| Cleavage | Days 1–4 | Cell division, cells get smaller |
| Morula | Day 4 | 16–32 cells, solid ball |
| Blastocyst | Day 5–6 | Cavity forms, ICM differentiated |
| Implantation | Days 6–9 | Trophoblast invades endometrium |
| Gastrulation | Weeks 2–3 | Three germ layers form |
| Organogenesis | Weeks 4–8 | Organ systems develop |
| Foetus | Week 9+ | Growth and maturation |
Common Mistake
Students often confuse the morula and the blastocyst, or omit the morula stage entirely. Remember the sequence: Zygote → (cleavage) → Morula → (fluid fills in) → Blastocyst. The morula is the solid ball; the blastocyst is the hollow ball. This distinction appears in NEET and CBSE 5-mark questions.
NEET frequently asks: “Where does fertilisation occur?” (ampullary region of fallopian tube); “What is implantation?” (embedding of blastocyst in endometrium); “At what stage does the embryo implant?” (blastocyst stage). These are 1-mark factual questions but easy to miss if you’re fuzzy on the sequence.