Body and organ size and composition during late foetal and postnatal development of rat.

Development induced deep anatomical changes and tissue composition alterations in the rat. To determine the extent of these changes, the organ weight and size of 19 and 21 day rat foetuses and of 1, 5, 10, 20 and 30 day old Wistar rat pups have been studied and compared with adults. Different tissues showed varying rates of cell and tissue growth as well as tissue cellularity during development. Tail length is not a good index of skeletal growth. Brain growth was much slower from late intrauterine life to adulthood than most other organs. Skin weight increased more than 3-fold between days 19 and 21 of intrauterine life. Striated muscle proportion to body weight remained practically constant throughout all postnatal life studied.