The Developing Human Clinically Oriented Embryology 8th Edition CHM
Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being. Although most developmental changes occur during the embryonic and fetal periods, important changes occur during later periods of development: infancy, childhood, adolescence, and early adulthood Development does not stop at birth. Important changes, in addition to growth, occur after birth (e.g., development of teeth and female breasts).
It is customary to divide human development into prenatal (before birth) and postnatal (after birth) periods. The main developmental changes occurring before birth are illustrated in the Timetable of Human Prenatal Development (Figs. 1-1 and 1-2). Examination of the timetable reveals that the most visible advances occur during the third to eighth weeks of embryonic development. During the fetal period, differentiation and growth of tissues and organs occur. The rate of body growth increases during this period.