a.
Embryonic period
Summary of the changes during embryonic and fetal development • Figure 16.15
Neural plate
Neural
groove
Cut edge of
amnion
Yolk sac
Developing
brain
Heart
prominence
Developing
spinal cord
Heart
prominence
Developing
upper limb
Tail
Developing
lower limb
2 0
-day embryo
24-day embryo
32-day embryo
1 -4 w e e k s
0.6 cm (3/16 in.),
weight minimal
Primary germ layers and notochord develop; neurulation occurs.
Blood vessel formation begins and blood forms; heart forms and begins to beat.
Chorionic villi develop and placental formation begins.
Eyes and ears begin to develop; body systems begin to form.
Developing ear
Developing nose
Upper limb
Lower limb
Umbilical cord
44-day embryo
Ear
Eye
Nose
Upper limb
Umbilical cord
Lower limb
52-day embryo
5-8 w e e k s
3
cm (1.25 in.),
1
g (1/30 oz)
Limbs become distinct and digits appear.
Heart becomes four-chambered.
F.mnjm f nrrrtittcl
I e
UMMUX
KE?
1
11 I Mil ; u
Bone formation begins.
Blood cells start to form in liver.
External genitals begin to differentiate.
Tail disappears.
Major blood vessels form.
Internal organs continue to develop.
494 CHAPTER 16
The Reproductive Systems
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