1. Fertilization

2. Fertilization
Joining of sperm & egg
sperm head enters egg

3. 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

4. Cleavage zygote  morula  blastula establishes future development

5. Organogenesis Umbilical blood vessels Mammalian embryo Chorion
Bird embryo
Amnion
Yolk
sac
Allantois
Fetal blood vessels
Placenta
Maternal blood vessels

6. Placenta
Materials exchange across membranes

7. Human fetal development
4 weeks
7 weeks

8. Sex determination Zygote Sperm Develop in early embryo Y Testes OvumXY X
SRY
Seminiferous
tubules
Indifferent
gonads
Leydig cells X
No SRY
Ovaries
Ovum XX
(Follicles do not
develop until
third trimester) X
Sperm
Zygote

9. Human fetal development
10 weeks

10. Human fetal development
12 weeks
20 weeks

11. 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

12. 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

13. Human fetal development
30 weeks (7.5 months)
umbilical cord

14. Getting crowded in there!!
32 weeks (8 months)
The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

15. Hormone induction
Birth
positive feedback

16. Birth (after 36 weeks) 2005-2006 Intestine Placenta Umbilical cord
Wall of
uterus
Bladder
Cervix
Vagina

17. The end of the journey!