Summary

There are 3 phases of embryological development

  1. Pre-embryonic period (0-3 weeks)
    • From Fertilisation => Trilaminar Disc Formation
    • Damage at this time causes death
  2. Embryonic period (4-8 weeks)
    • Organogenesis (organ formation)
    • Damage causes major morphological abnormalities
  3. Foetal period (9 – 40 weeks)
    • Sex differentiation and organ growth
    • Damage causes minor morphological abnormalities

This table describes a summary of the timeline of development.

Week 1

    • Cleavage divisions
    • Formation of blastocyst

Week 2

    • Formation fo blastula, bilaminar disk, amnion, and yolk sac
    • Implantation (blastocyst implants into uterine wall)

Week 3

    • Gastrulation
    • Primitive streak formation (first sign of axis formation)
    • Notochord formation(induces ectoderm à nervous system), Neural plate, Somites

Week 4

    • Cylindrical embryo (coelomic cavities): major event / change in shape
    • Head and tail folds
    • Heartbeat (21-22 days)
    • Limb buds (limbs)
    • Complete neural tube
    • Branchial arches

Weeks 5-8

Organ formation occurs

    • Heart
    • Eyes
    • Brain
    • Hands, feet
    • GI tract
    • GU tract (urinary)
  • Primordia of all major organs are present at this stage

Weeks 8-38

Foetal period

    • Organ growth +maturation
    • Certain organs develop later than others

      • Kidneys
      • Lungs
      • Brain
      • Sex specific external genitalia
This image describes the process graphically (Moore, 1993)