Question
Describe the stages of embryonic development from fertilisation to gastrulation in humans. What is the significance of each stage — zygote, cleavage, morula, blastocyst, and gastrulation?
(NEET 2023, similar pattern)
Solution — Step by Step
After fertilisation, the zygote (2n) undergoes rapid mitotic divisions called cleavage. These divisions are special:
- The cells divide but don’t grow — the overall embryo size stays the same as the original zygote
- Each daughter cell is called a blastomere
- First cleavage produces 2 cells, then 4, 8, 16, and so on
Cleavage occurs as the embryo travels down the fallopian tube toward the uterus (takes about 5-7 days).
At about 16-32 blastomeres, the embryo becomes a solid ball of cells called the morula (Latin for mulberry — it looks like one).
The morula is still surrounded by the zona pellucida (protective coat from the oocyte). At this stage, all cells look similar and haven’t differentiated yet.
The morula develops a fluid-filled cavity called the blastocoel. Now it’s called a blastocyst (the mammalian equivalent of a blastula):
- Trophoblast — the outer layer of cells; will form the placenta and help in implantation
- Inner cell mass (ICM) — a cluster of cells on one side; will form the embryo proper
- Blastocoel — the fluid-filled cavity
The blastocyst implants into the uterine wall (endometrium) around day 7 — this is called implantation.
After implantation, the inner cell mass reorganises into three primary germ layers:
| Germ layer | Position | Will form |
|---|---|---|
| Ectoderm | Outer | Skin, nervous system, sense organs |
| Mesoderm | Middle | Muscles, bones, blood, kidneys, heart |
| Endoderm | Inner | Gut lining, liver, pancreas, lungs |
Gastrulation involves cell migration and folding — it converts the flat disc of cells into a three-layered structure. This is one of the most critical events in development because it establishes the body plan.
Why This Works
Each stage of embryonic development serves a specific purpose. Cleavage rapidly increases cell number without growth — this distributes the zygote’s cytoplasmic contents among many cells. The morula compacts its cells, establishing tight cell-cell connections essential for later differentiation. The blastocyst separates into two cell populations (trophoblast for nutrition, ICM for the embryo), and gastrulation assigns each cell to a developmental fate.
The sequence is remarkably conserved across animals — from frogs to humans, the basic stages (cleavage → morula → blastula → gastrula) follow the same pattern.
Alternative Method — Timeline Summary
For NEET, memorise the human embryonic timeline:
- Day 1: Fertilisation → zygote
- Days 2-4: Cleavage → morula (in fallopian tube)
- Days 5-6: Blastocyst formation (still in fallopian tube/entering uterus)
- Day 7: Implantation into endometrium
- Week 2-3: Gastrulation (three germ layers form)
- Week 4 onwards: Organogenesis (organs begin to form)
Common Mistake
Students confuse blastula and blastocyst. In general embryology, the hollow ball stage is called a blastula. In mammals specifically, it’s called a blastocyst because it has a distinct ICM (not seen in most other animals). For NEET, use “blastocyst” when discussing human embryology. Also, don’t say “the morula implants” — implantation happens at the blastocyst stage, not the morula stage. The morula is still in the fallopian tube.