Abstract
Factors that influence the formation and volumes of embryonic fluids in amphibia, birds, and mammals are reviewed. Some bodies of fluids segregate discretely only in embryonic and fetal stages; others form permanently in cavities or interstitial compartments. Major volume changes during development are described, especially for blastocoel, archenteron, amniotic, allantoic, and interstitial fluids. Epithelial layers secrete fluids into sealed cavities. The rates of secretion per unit of surface resemble one another to a surprising degree; in large species, prolongation of activity yields the larger volumes. Secretion in some surfaces may cease at a predetermined age; in others, cell deaths allow fluids to disperse. Thus redistributions of fluids change with age both absolutely and relately to their sum. Replacement or regeneration of cavity fluids has not yet been clearly demonstrated. Embryonic sacs appear to secrete nearly isotonic fluids in accordance with intrinsic programs of the cells constituting the sacs.
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