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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.
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