Biology 322Human Anatomy
Human Embryology
One of the best ways to understand the structure ofthe adult human body is to understand how itdeveloped.
Similarly:
Human Development: 3 Periods
Pre-embryonic:
Embryonic:
Fetal:
Fertilization:
Resulting cell =
As
Fluid-filled
Pre-embryonic period now over,
Embryonic period begins.
Fertilization – 2 wks = Pre-embryo
2 wks – 8 wks = Embryonic period
8 wks – birth = Fetal period
First:
View From Top,Showing Ectoderm
View From Edge (Cross-section)Showing All Three Layers
As embryo develops:
The first system to begin developing
~ Day 22 – 23,
By day 28:
~ Day 28: begin developing.
Length ~ 4mm(width of pencil)
Late embryonic period (4 – 8 weeks):
Nervous system continues to develop;
brain rapidly enlarges and folds
Eyes and ears begin to develop
Digestive system forms
Heart folds, divides into chambers, begins contractions
Limbs continue developing, fingers and toes separate
Lungs bud off from digestive system and grow
Kidneys, bladder, gonads develop
Face develops as two halves on side of head; move tofront and fuse
End of embryonic period
• Body has human shape
• Almost all organs have begun developing
• Head growth still most pronounced
• Limbs complete
• Length ~
• Mass ~
Beginning of fetal period
(8 weeks):
Mass of quarter: 5.7 grams
Placenta well developed:
In placenta:
Ninth week:
(Organs and systems continue development)
Fetus =
Tenth week:
(Organs and systems continue development)
Fetus =
After tenth week:
(Organs and systems continueto develop)
After 17 weeks (four months)
Skin formed, but thin. Blood vessels easilyseen through it
Hair starting to develop. Fingernails andtoenails forming.
Eyelids still fused shut.
Ovaries form primordial follicles.
May suck thumb.
Moves arms, legs, head
Mother can feel movement.
After 26 weeks (six months):
Hair present over entire body. Willbecome longer on scalp
Eyebrows and eyelashes forming
Eyelids almost open
Lungs producing surfactant
Testes begin descent through inguinalcanal
Swallowing amniotic fluid
Movement of hands, feet, face
After 34 weeks (eight months):
Skin pink, smooth
Longer hair on head
Eyes open, respond to light
Fingernails and toenails havereached ends of fingers and toes
Testes have reached scrotum
Fingers can grasp objects, finemovement of face and eyes
Nine months:
Ready for birth
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