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- (2 weeks, 1 day):
3 germ layers
Bilateral symmetry
- (2 weeks, 2 days):
Amnion with two cell layers
Secondary villi
Erythroblasts in yolk sac
Three types of blood-forming cells
- in yolk sac
- (2 weeks, 3 days):
Portion of the thyroid
- (2 weeks, 4 days):
Connecting stalk
Neural plate
Foregut, midgut, and hindgut
Brain is first organ to appear
- (2 weeks, 6 days):
Somites 1 - 3 pairs of somites
Forebrain, midbrain, and hindbrain
Neural crest
Neural groove deepens substantially
Otic disc
Stomodeum forming
Blood vessels appear in yolk sac,
- body, and amnion
Dorsal aortae (paired)
Heart
Paired tubular heart
Foregut
Numerous blood-forming locations in
- yolk sac
- (3 weeks):
Blood cells
Blood vessels
- Less Events...
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| Fertilization |
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| Birth |
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- (3 weeks, 1 day):
Body cavities
Folding of the embryo begins
Neural tube
Hyoid arch
Atria (right and left) far apart
Circulatory system function begins
Endocardial tubes fuse forming
- tubular heart
Heart begins beating
Pericardium
Sinus venosus
Tubular heart begins folding
Umbilical arteries
Umbilical veins (right and left)
Respiratory primordium
Midgut emerging
- (3 weeks, 3 days):
Neuropore (near brain) closes
Notochord
Membrane between future mouth and
- throat may begin to rupture
Cystic primordium
Liver
Thyroid complete
- (3 weeks, 5 days):
21 - 29 pairs of somites
Progressively C-shaped embryo
Brain involves 40% of neural tube
Lowermost spinal cord formation
- begins
Neuropore (lower back) closes
Pharyngeal arches 1, 2, and 3
Descending aorta
Internal carotid arteries
Unidirectional circulation
Lung bud
First part of pancreas
Earliest hint of upper limbs
- (4 weeks):
Amnion surrounds embryo
Umbilical cord
Cerebellum
Fourth ventricle
Most cranial nerve ganglia
Circulatory system "well
- established"
Functioning two-chamber heart
Heart chambers bulging with fluid
Heart now functions as two parallel
- pumps
Heart rate (about) 113 beats/min
Bronchial buds
Lungs begin filling chest cavity
Right and left lung buds
Trachea or windpipe
Esophagus and stomach
Intestines growing in length
Pharynx
Small & large intestines
Ventral pancreas
Upper & lower limb buds
Upper and lower limb buds
Periderm
Skin transparent
- Less Events...
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- (34 weeks):
Rapid weight gain
- (35 weeks):
Amniotic fluid volume peaks
Firm grip
- (36 weeks):
Surfactant production accelerates
Brain weight 300 grams
Crown-heel length 48.5 cm
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