About the time of implantation, certain cells within the blastocyst (an early stage of embyronic development) organize themselves into a group that will give rise to the body of the offspring. 

This marks the beginning of the embryonic period of development. The offspring is called an "embryo" until the end of the eighth week, after which it is a "fetus." Eventually, the outer cells of the embryo together with cells of the maternal endometrium (wall of the uterus) form a complex vascular structure called the "placenta." 

This organ serves to attach the embryo to the uterine wall, to exchange nutrients, gases, and wastes between the maternal blood and the embryonic blood, and to secrete hormones.