Remember, the placenta is the organ that eventually begins the exchange of nutrients and gases between the mother and that fetus. Now, once implantation takes place, we also begin the process of gastrelation. Now remember, gastrolation is the process by which we form the three different germ layers. And these three different germ layers eventually give rise to the different tissues, organs and systems that are found within that adult individual. Now, this all takes place within the first trimester. In fact, it takes place within the first several weeks following fertilization.