A little picture is worth a million words. Chinese proverb Prenatal human development is an extremely complicated process related to genetics, biochemistry, anatomy, and physiology. There are no developmental changes, either chemical or morphologic, without simultaneous changes in molecular organization. The astonishing buildup ofbiostructures always precedes their proper function. The development of an embryo is genetically coded and is based on interactions related to the selective switching on and off of genes. Interactions are cell-to-cell mediated, mediated by extracellular fluids, or mediated by special pathways. Every substance involved in developmental is to be recognized by its target. interactions, before triggering a metabolic or a morphogenic event, Complex physical and immunologic recognitions are involved in the process of differentiation. Small pieces of evidence are collected to create a mosaic picture elucidating the development. This picture is fascinating and represents the biggest biological puzzle: the puzzle of development. There is no doubt that analysis of human prenatal development is a basis for understanding normal and pathologic relationships between structure and function. Today, there are approximately 2000 different inborn congenital anomalies and syndromes. This book attempts to present a complete realistic account of human morphogenesis, the differentiation of structures, using direct photographs of normal specimens obtained from legal medical abortions of unwanted pregnancies. Emphasis has been placed on contemporary techniques: histochemistry and scanning electron microscopy. The text is as simple as possible; meticulous detailed anatomic descriptions have been omitted.