1. Umbilical Cord: The umbilical cord is the lifeline connecting the fetus to the placenta. It contains blood vessels that transport nutrients and oxygen to the fetus and waste products away. After birth, the umbilical cord is severed, leaving a scar called the navel or belly button.
2. Placenta: The placenta is a temporary organ that facilitates the exchange of gases, nutrients, and waste products between the fetus and the mother. After birth, the placenta is expelled from the mother's body as the afterbirth.
3. Allantois: The allantois is a sac-like structure that functions as a waste storage organ for the fetus. It contributes to the formation of the umbilical cord and later regresses, forming parts of the bladder and urinary tract.
4. Ductus Arteriosus: This blood vessel connects the pulmonary artery to the aorta in the fetus, bypassing the lungs as they are not yet functional. After birth, the ductus arteriosus constricts and closes, becoming the ligamentum arteriosum.
5. Foramen Ovale: This opening in the septum between the right and left atria of the heart allows blood to bypass the pulmonary circulation in the fetus. After birth, the foramen ovale closes due to increased pressure in the left atrium, forming the fossa ovalis.
6. Ductus Venosus: This blood vessel connects the umbilical vein to the inferior vena cava, bypassing the liver in the fetus. After birth, the ductus venosus constricts and closes, becoming the ligamentum venosum.
7. Fetal Membranes: The amniotic sac and chorion, which surround the fetus, protect and support it in the womb. After birth, these membranes are expelled as part of the afterbirth.
8. Yolk Sac: Though not as prominent in pigs as in other species, the yolk sac in fetal pigs contributes to early development and eventually degenerates.
9. Branchial Arches: These structures are present in the early development of all vertebrates, giving rise to various facial structures like the jaws, ears, and throat. In fetal pigs, they are visible externally as bulges on the neck, but they regress and form various structures during development.
10. Hair: Fetal pigs have a thin layer of hair called lanugo that is shed before birth.
It's important to note that these structures may not completely disappear in all individuals. Some remnants of these structures can persist, but they are no longer functional.