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- (2 weeks, 1 day):
3 germ layers Rostral-caudal orientation- (2 weeks, 2 days):
Erythroblasts in yolk sac Three types of blood-forming cells in- yolk sac
Amnion with two cell layers Secondary villi- (2 weeks, 4 days):
Foregut, midgut, and hindgut Brain is first organ to appear Neural plate Connecting stalk
- (2 weeks, 6 days):
Numerous blood islands in umbilical- vesicle
Foregut Stomodeum forming Beginnings of the heart can be seen Blood vessels emerge simultaneously in- umbilical vesicle, embryo proper, amnion,
- and connecting stalk
Dorsal aortae (paired) Paired tubular heart Otic disc Forebrain, midbrain, and hindbrain Neural crest Neural groove deepens substantially Three main divisions of brain Somites: 1 - 3 pairs- (3 weeks):
Blood and blood vessels- Less Events...
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- (3 weeks, 1 day):
Midgut emerging Respiratory outgrowth Atria (right and left) far apart Circulatory system function begins Endocardial tubes fuse forming tubular- heart
Heart begins beating Pericardium Primary head vein Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins (right and left)
Neural tube Body cavities Hyoid arch- (3 weeks, 3 days):
Thyroid complete Cystic primordium Liver Membrane between future mouth and throat- may begin to rupture
Neuropore (near brain) closes Notochord- (3 weeks, 5 days):
First part of pancreas Pharyngeal arches 1, 2, and 3 Lung bud Descending aorta Internal carotid arteries Unidirectional circulation Brain involves 40% of neural tube Lowermost spinal cord formation begins Neural tube closes (lower back) Somites: 21 - 29 pairs Upper limb primordium at level of- somites 8 to 10
Progressively C-shaped embryo- (4 weeks):
Periderm Skin is so thin, you can see through it! Esophagus primordia Intestines growing in length Pancreas: Ventral pancreas Pharynx Small & large intestines Bronchial buds Lungs begin filling chest cavity Right and left lung buds Trachea 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 Cerebellum Fourth ventricle Most cranial nerve ganglia Amnion surrounds embryo Limb buds - the first sign of arms and- legs
Lower limb buds Umbilical cord emerging Upper and lower limb buds- Less Events...
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- (7 weeks, 1 day):
Upper limbs with slightly flexed elbows Sacrocaudal spinal cord formation- (secondary neurulation) complete
- (7 weeks, 1 day - 8 weeks):
Folds in stomach wall- (7 weeks, 2 days):
Arteries and veins of heart complete- (7 weeks, 3 days):
Cartilaginous nasal septum Wrists slightly flexed Eyelids grow rapidly Cerebral hemispheres cover more than- half of diencephalon
- (7½ weeks):
Hands begin to touch face The hands touch each other as do the- feet!
Fingertips thicken Plantar pads toes The knee joints have arrived EKG pattern similar to adult- (7 weeks, 4 days):
The fingers are free- (7 weeks, 5 days):
Bone-forming cells called osteoblasts- emerge
Hands can reach one another and fingers- can overlap
Cerebral hemispheres cover 75% of- diencephalon
Cortical plate expanding rapidly Internal capsule with connections to- epithalamus, dorsal thalamus, and
- mesencephalon
- (7 weeks, 6 days):
The toes are free- (8 weeks):
Complex response to touch More frequent hand-to-face contact Mouth opens & closes Squinting The embryo floats and rolls over in the- womb
Hair in eyebrows & around mouth Skin multi-layered, loses transparency Male embryos are making testosterone- already!
The embryo's joints are similar to adult- joints
Diaphragm complete Urethra Urine production and release Longitudinal muscles in esophagus Peristalsis in large intestine Occasional breathing motions begin Blood supply to the brain closely- resembles adult pattern
Superior vena cava Cranial nerve distribution mimics adult- pattern
Retina: Four of the ten adult layers- present
Tympanic membrane "The hindbrain "presents striking- resemblance to that of the newborn."
Brain represents 43% of embryo Grey and white matter Right- and left-handedness emerges Crown-heel length 4.3 cm Embryo contains approximately 1 billion- (10^9) cells
Embryonic Period Ends The embryo has more than 4,000 parts- (adults have about 4,500)
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- (34 weeks):
Rapid weight gain-
- (35 weeks):
Firm grip Amniotic fluid volume peaks-
- (36 weeks):
Surfactant production accelerates Brain weight 300 grams Crown-heel length 48.5 cm
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