Fertilization
AP Biology
2005-2006
Fertilization
 Joining of sperm & egg

AP Biology
sperm head enters egg
2005-2006
Cleavage
 Repeated mitotic divisions of zygote
1st step to becoming multicellular
 unequal divisions establishes body plan

 different cells receive different portions of egg
cytoplasm & therefore different regulatory signals
AP Biology
2005-2006
Cleavage
 zygote  morula  blastula

AP Biology
establishes future development
2005-2006
Organogenesis
Mammalian embryo
Umbilical blood vessels
Chorion
Bird embryo
Amnion
Yolk
sac
Allantois
Fetal blood vessels
Placenta
AP Biology
Maternal blood vessels
2005-2006
Placenta
 Materials exchange across membranes
AP Biology
2005-2006
Human fetal development
4 weeks
AP Biology
7 weeks
2005-2006
Sex determination
Sperm
Ovum
Y
Zygote
XY
X
SRY
Indifferent
gonads
No SRY
X
Ovum
AP Biology
X
Sperm
Develop in
early
embryo
Testes
XX
Zygote
Seminiferous
tubules
Leydig cells
Ovaries
(Follicles do not
develop until
third trimester)
2005-2006
Human fetal development
10 weeks
AP Biology
2005-2006
Human fetal development
12
weeks
AP Biology
20 weeks
2005-2006
Human fetal development
 The fetus just spends much of the 2nd &
3rd trimesters just growing
…and doing various flip-turns & kicks
inside amniotic fluid
Week 20
AP Biology
2005-2006
Human fetal development
24 weeks (6 months; 2nd trimester)
fetus is covered
with fine, downy
hair called
lanugo. Its skin
is protected by
a waxy material
called vernix
AP Biology
2005-2006
Human fetal development
30 weeks (7.5 months)
umbilical cord
AP Biology
2005-2006
Getting crowded in there!!
32 weeks (8 months)
The fetus
sleeps 90-95%
of the day &
sometimes
experiences
REM sleep, an
indication of
dreaming
AP Biology
2005-2006
Hormone
induction
positive feedback
AP Biology
Birth
2005-2006
Birth (after 36 weeks)
Intestine
Placenta
Umbilical
cord
Wall of
uterus
Bladder
AP
Cervix
Vagina
Biology
2005-2006
The end of the journey!
AP Biology
2005-2006