EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
A human embryo after 7 weeks development, photographed following a spontaneous miscarriage. The head dominates the image; also visible is a primitive hand and the large red mass at right is the heart. After 7 weeks the formative or embryonic period is regarded as coming to an end. During this early period tissue differentiation and organ development tend to overshadow the growth that accompanies these events. In the subsequent fetal period the accent is on growth rather than differentiation, but the latter continues throughout this period and after birth (and in some tissues, throughout life).
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