Embryonic Development
The embryonic stage extends from the second week by the 8th week and is charactrised by formation of placenta, the development of internal organs and appearance of the major external body structures. Throughout this period the embryo takes on a human shape by morphogenetic processes. The rudiments of several organs get established during the 3rd week but further development will occur from then on to 8th week by organogenesis, a process you are already familiar with. Due to the simultaneous developmental changes taking place during this period the developing embryo is specifically sensitive to teratogens, that is, certain agents like alcohol or drugs etc. can induce malformations in the fastly forming tissues and organs.