Introduction
The terminology used to describe the development of the conceptus from conception to term is variable in the literature. From studies in radiobiology, where the effects of in utero exposure to ionizing radiation have been studied, two main phases of development may be considered: the embryonic period and the fetal period. The term embryo refers to the initial stages of growth and development up to the age at which most of the body organs have formed, considered to be at the end of the 8th week (56 days) post conception.
The subsequent period of fetal development comprises principally growth and maturation of organs. It continues up to birth at 38 weeks post conception (266 days) when the body mass is taken to be 3.5 kg. Although the morphological appearance of the embryo during the first 3 weeks of development post conception does not seem very structured, the pattern of the basic body plan is already established at this time (Potter, Craig, 1975).
Thus, the dorsal ectodermal cells proliferate and differentiate to form the neural plate, which develops into the neural tube, which comprises the nervous system. The mesodermal germ layer develops, for example, into the circulatory system and the heart and the endodermal layer forms the digestive system. By the end of the 4th week of gestation the embryo, which is still only about 4 mm long, has established the basis of most of the maturing organ systems. The limbs are still absent and the urogenital system has developed only the early traces of the embryonic kidneys. The period of organogenesis (organ formation) may be considered to last up to about the end of the second month (taken here to be 56 days of gestation), at which time the developing embryo still weighs less than 10 g...................
