Summary

There are 3 phases of embryological development

Pre-embryonic period (0-3 weeks)
- From Fertilisation => Trilaminar Disc Formation
- Damage at this time causes death
Embryonic period (4-8 weeks)
- Organogenesis (organ formation)
- Damage causes major morphological abnormalities
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