Fig. 9. A hypothetical model accounting for steady state water and salt turnover, hyper-osmoregulation and silver staining of post-gastrula embryos of Hemigrapsus. Active uptake of sodium and chloride takes place across the embryonic ectoderm into a hyperosmotic extracellular compartment by bounded embryonic and extra-embryonic ectoderm. Continuous osmotic entry of water into the extracellular compartment generates a small internal hydrostatic pressure causing both water and salts to leak out between the cells. In embryos vital stained with AgNO₃, Cl^– ions are precipitated as AgCl between the outer and inner membranes. After gastrulation this paracellular flow occurs primarily in the region of the embryonic dorsal organ, which therefore functions as a simple filtration-type excretory organ.