1. Fertilization & Early Development

2. zygote

3. morula
Solid ball of cells 3 days after fertilization

4. blastocyst
Morula continues cell division and hollows out to form a blastocyst (day 5 – week 2)

5. trophoblast
The cells that form the wall of the blastocyst

6. inner cell mass
Eventually becomes the developing offspring

7. embryo / gastrula
Embryo is termed a gastrula at the end of the second week

8. fetus

9. placenta

10. maternal portion
The area of the uterine wall (decidua basalis) where the chorionic villi attach

11. fetal portion
Includes the chorion and chorionic villi that extends into the uterine wall

12. Chorion (maroon)

13. chorionic villi

14. Amnion (white)

15. amniotic cavity (space)

16. yolk sac

17. Allantois (future umbilical cord)

18. Fetal circulation

19. ductus arteriosus
Connects the pulmonary trunk to the descending portion of the aortic arch
Becomes ligamentum arteriosum

20. ductus venosus
Connects the umbilical vein directly to the inferior vena cava, bypassing the liver
Becomes the ligamentum venosum

21. foramen ovale
An opening in the atrial septum where blood passes from the right atrium directly to the left atrium and bypassing the lungs
Becomes fossa ovalis

22. umbilical cord

23. umbilical arteries (blue)

24. umbilical vein (red)