The remainder of the embryonic period, weeks 4 to 8, are particularly important. They are the weeks during which the embryo develops a human body form and its organs develop significantly. Consequently, they are also the weeks during which development can go badly wrong if normal processes are disturbed.
Morphogenesis refers to the development of shape or form.
It includes the differentiation of the organ's cells and tissues and the development of the pattern of their arrangements that is characteristic of the organ. It also includes development of function. Because the beginnings of most essential external and internal structures are formed during the fourth to eighth weeks, this is the most critical period of development. During week 4 the three-layered embryo takes on the shape and general appearance that is characteristic of the vertebrate body plan. During the second half of the embryonic period the embryo develops distinctively human features.