Osmotic activity of tissues during fetal and post-natal growth.

The osmotic pressure maintained by liver tissue of the white rat preceding birth is less than that of the maternal blood serum and shortly after birth approximates this level. Following birth osmotic pressure of liver tissue, continuing to increase, reaches after about 60 to 90 days the level found in the liver of mature animals and is then isotonic with solutions of sodium chloride with concentration slightly more than twice that isotonic with blood serum. Osmotic pressure maintained by kidney tissue pursues with growth a similar course but at a lower level and about 35 to 60 days after birth reaches that found in the mature animal being represented by isotonicity with a concentration of sodium chloride slightly less than twice that isotonic with blood serum. The tissues of the whole fetus are isotonic with sodium chloride solutions less concentrated than that isotonic with the maternal blood serum.