During this relatively short embryonic period (weeks 4 to 8), one sees the beginnings of all major internal and external structural (organ and organ systems) develop during which time the 3 germ layers give rise to specific tissues and organs – the period of organogenesis. The shape of the embryo changes, and major features of the external body form (morphogenesis) become recognizable by the end of month 2. In addition, major congenital malformations can occur due to exposure of the embryo to teratogens during this developmental period.

Week 4 of development

◦ABOUT DAYS 22 TO 23: embryo is almost straight or slightly curved, and somites create conspicuous surface elevations. The neural tube is closed opposite the somites but is open at its caudal and rostral neuropores

◦ABOUT DAY 24: the first (mandibular) and second (hyoid) branchial arches become distinct

◦ABOUT DAY 26: 3 pairs of branchial arches are seen and the rostral neuropore closes

◦ABOUT DAY 28 (END OF WEEK 4): the fourth pair of branchial arches and the leg buds are seen

Week 5 of development

There are fewer body form changes this week

◦HEAD GROWTH is accelerated as a result of rapid brain development

Week 6 of development

1. LIMB BUDS, especially the forelimbs, show regional differentiation. Hind limbs develop later

2. SOME SMALL SWELLINGS appear around the groove between the first branchial arches. The groove becomes the external auditory meatus, the swellings the ear auricle

3. THE EYE becomes obvious due to appearance of retinal pigment

4. THE HEAD appears larger relative to the trunk and bends even farther over the heart prominence

5. SOMITES are visible in the lumbosacral region by the middle of this week

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