The placenta and the cord
After implantation, cell division continues further and development of the embryo takes place.
- At about six weeks, protective membranes called the amniotic membranes develop. A fluid fills the space within them.
- The embryo at about nine weeks of age is called the foetus. The foetus is thus submerged in that fluid.
The placenta
The amniotic membranes of the foetus and the inner wall of the uterus join together and form the placenta. Nutrients and oxygen contained in the mother's blood diffuse into this placenta.
The umbilical cord
Transporting to the foetus the nutrients and oxygen that have diffused into the placenta, and removing the waste products produced by the foetus, both take place along the umbilical cord.
The umbilical cord is a piece of tissue containing an artery and a vein, which makes the connection between the mother and the foetus in mammals that have a placenta.
What crosses, and what does not
No exchange of blood takes place across the umbilical cord. From the mother to the foetus pass nutrients, oxygen and also disease-causing agents — even disease-causing micro-organisms, some viruses among them, can travel across. Excretory substances, carbon dioxide and the like leave the foetus across this same umbilical cord.
Figure to be added
Two circulations, one exchange surface
The mother's blood and the foetus's blood never mix — they run close enough for substances to diffuse between them and no closer. That is why a foetus can have a different blood group from its mother, and also why anything small enough to diffuse, harmful or not, reaches it.