A 28-year-old pregnant woman at 8 weeks of gestation undergoes ultrasound examination. The embryo is in the trilaminar disc stage. Which of the following germ layers gives rise to the nervous system?

Explanation

## Germ Layer Derivatives in Early Embryogenesis At the trilaminar disc stage (weeks 2–3 of human development), three primary germ layers are present: ### Ectoderm - **Outermost layer** - Gives rise to: - **Nervous system** (brain, spinal cord, peripheral nerves) via neurulation - Epidermis of skin - Hair, nails, tooth enamel - Sensory organs (eye, ear) - Cranial and spinal nerves ### Mesoderm - **Middle layer** - Gives rise to: - Skeletal and smooth muscle - Bone and cartilage - Connective tissue - Blood and lymphatic vessels - Heart - Kidneys and gonads ### Endoderm - **Innermost layer** - Gives rise to: - Gastrointestinal tract lining - Respiratory tract - Liver, pancreas - Thyroid, parathyroid glands **Key Point:** The ectoderm is the source of the entire nervous system through the process of neurulation, which begins around week 3. **High-Yield Mnemonic:** **"ENDO = ENDOderm (GI tract)"** and **"ECTOderm = ECTOpia (nervous system outside the body)"**