1. Survey of Embryonic Development

2. Embryo Fertilization 1-week conceptus 2-week conceptus 3-week embryo
Diagrams showing the size of a human conceptus from fertilization to the early fetal stage
Embryo
Fertilization
1-week conceptus
2-week conceptus
3-week embryo
4-week embryo
5-week embryo
6-week embryo
7-week embryo
8-week embryo
9-week fetus
12-week fetus

3. Sperm penetration and the cortical reaction (slow block to polyspermy
Corona radiata
Zona pellucida
First polar body
Oocyte in
second meiotic
division
(b)
Sperm nucleus
Acrosome
Acrosomal
reaction
Granulosa cells
of corona radiata
Zona pellucida
Extracellular
space
Oocyte plasma
membrane
Fusion of oocyte
and sperm plasma
membranes
Cortical
reaction
Cortical granule
Oocyte cytoplasm
Sperm nucleus engulfed
by oocyte cytoplasm
(a)

4. Events immediately following sperm penetration
Extracellular space
Daughter
cells
Second meiotic division
of oocyte
Second meiotic division
of first polar body
Detached
sperm tail
(g)
(a)
(h)
Sperm pro-
nucleus
Corona
radiata
Ovum
nucleus
Anaphase of
first cleavage
division
Zona
pellucida
Polar
bodies
(b)
(f)
Sperm and ovum
chromosomes
Male pronucleus
Mitotic spindle
Centriole
Female pronucleus
(e)
(c)
Zygote
(d)

5. Cleavage from zygote to blastocyst
Inner cell
mass
Degenerating
zona
pellucida
Blastocyst
cavity
Blastocyst
cavity
Trophoblast
(a)
Zygote
(fertilized
egg)
(b)
4-cell stage
2 days
(c)
Morula
3 days
(d)
Early blastocyst
4 days
(e)
Implanting blastocyst
6 days
Sperm
(b)
Fertilization
(sperm meets egg)
(a)
(c)
Uterine tube
Ovary
Oocyte
(egg)
(d)
Uterus
(e)
Ovulation
Endometrium
Cavity of
uterus

6. Hormonal changes during pregnancy
Human chorionic
gonadotropin
Estrogens
Relavtive blood levels
Progesterone 4 8 12 16 20 24 28 32 36
Gestation (weeks)
Ovulation
and fertilization
Birth

7. Implantation of the blastocyst
Endometrium
Uterine endometrial
epithelium
Inner cell mass
Trophoblast
Blastocyst cavity
Lumen of uterus
(a)
Endometrial stroma
with blood vessels
and glands
Syncytiotrophoblast
Cytotrophoblast
Inner cell mass
(future embryo)
Lumen of uterus
(b)
(c)

8. Formation of the three primary germ layers
Amniotic sac
Bilayered
embryonic disc
Yolk sac
(b) turned 90˚
(c) 3-D view
(d) Section view
in (e)
Amniotic sac (amnion)
Yolk sac
Left
Future opening
between mouth
and pharynx
Cut edge
of amnion
Tail
Head
(a) Embryo
(See Figure 28.7b)
Yolk sac
Primitive
streak
Epiblast
Hypoblast
(e) Bilayered embryonic disc,
lateral-superior view
Right

9. Formation of the three primary germ layers
Head
Yolk sac
(cut edge)
Primitive streak
Cut edge
of amnion
Epiblast
Hypoblast
Left
Right
Endoderm
(g) 14–15 days
Sections
Ectoderm
Primitive
streak
Tail
(f) Bilayered embryonic
disc, superior view
(h) 16 days
Mesoderm
Definitive
endoderm

10. Head Neural plate Surface ectoderm Future neural crest Amnion
Neurulation
Head
Neural
plate
Surface
ectoderm
Future
neural
crest
Amnion
Amniotic cavity
Neural plate
Left
Right
Ectoderm
Notochord
Mesoderm
Cut
edge of
amnion
Primitive
streak
Notochord
Endoderm
Tail
(a) 17 days
Yolk sac
Cut edge
of amnion
Neural
groove
Neural groove
Neural
plate
Neural
fold
Neural
crest
Primitive
node
Notochord
Primitive
streak
(b) 19 days

11. Tissues derived from the three germinal layers
Ectoderm: nervous tissues, epidermis and melanocytes.
Mesoderm: bones, muscles, mesenchyme, reproductive system, urinary system.
Endoderm: digestive system, respiratory system, endocrine system.

12. Amniotic cavity Endometrium Lacuna (intervillus space) containing
Events of placentation, early embryonic development, and extraembryonic membrane formation
Amniotic cavity
Endometrium
Lacuna (intervillus
space) containing
maternal blood
Primary
germ layers:
Chorionic villus
• Ectoderm
Maternal
blood vessels
Chorion
• Mesoderm
Proliferating
syncytiotrophoblast
Amnion
• Endoderm
Forming
body stalk
Cytotrophoblast
Amniotic cavity
Yolk sac
Bilayered
embryonic
disc:
Allantois
Extraembryonic
mesoderm
• Epiblast
• Hypoblast
Chorion
being formed
Endometrial
epithelium
Extra-
embryonic
coelom
Lumen of uterus
(a) 71/2-day implanting
blastocyst
(b) 12-day implanted
blastocyst
(c) 16-day embryo

13. Embryonic membranes
Amnion: encases and protects embryo against mechanical and temperature trauma.
Yolk sac: transfers nutrients to embryo. Replaced by placenta in humans.
Allantois: repository of embryo wastes. Replaced by placenta in humans.

14. Decidua basalis Maternal blood Chorionic villus Umbilical blood
Events of placentation, early embryonic development, and extraembryonic membrane formation
Decidua basalis
Maternal blood
Chorionic villus
Umbilical blood
vessels in
umbilical cord
Amnion
Amniotic cavity
Yolk sac
Extraembryonic
coelom
Chorion
Lumen
of uterus
Decidua
capsularis
(d) 41/2-week embryo

15. Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
Events of placentation, early embryonic development, and extraembryonic membrane formation
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical cord
Uterus
Decidua
capsularis
Lumen of
uterus
Extraembryonic
coelom
(f)
13-week fetus

16. Maternal arteries Maternal veins Myometrium Stratum basalis of
Events of placentation, early embryonic development, and extraembryonic membrane formation
Maternal
arteries
Maternal
veins
Myometrium
Stratum basalis of
endometrium
Maternal portion
of placenta
(decidua basalis)
Chorionic villus
containing fetal
capillaries
Fetal portion of
placenta (chorion)
Maternal blood
in lacuna
(intervillus space)
Fetal arteriole
Umbilical arteries
Fetal venule
Umbilical vein
Amnion
Connection to
yolk sac
Umbilical cord
(g)