The role of ion transport in the formation of sub-embryonic fluid by the embryo of the Japanese quail.

1. Using a modification of the method of New (1955) explanted blastoderms of the Japanese quail were cultured for up to 72 h, and the role of ions in the formation of sub-embryonic fluid (SEF) investigated. 2. Culture media deficient in either sodium or chloride ions reduced the volume of SEF secreted by up to a quarter. Ionic composition of the fluid was little altered, and sodium was transported against a concentration gradient. 3. Amiloride, an inhibitor of Na+/H+ exchange, reduced SEF production by half, whereas ionic composition and osmolality of the fluid was unchanged. 4. Likewise, acetazolamide, an inhibitor of carbonic anhydrase, reduced SEF production by the blastoderm, and left the ionic composition and osmolality of the fluid unaltered. 5. Neither furosemide or 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid at the concentrations used changed the volume of SEF formed or its ionic composition. 6. It is concluded that the secretion of SEF is dependent upon the active transport of sodium across the blastoderm; an amiloride-sensitive Na+/H+ exchanger and carbon dioxide hydration catalysed by carbonic anhydrase are also involved. 7. Furthermore, it is proposed that fluid transport across the blastoderm is the result of local osmotic gradients, not from a sub-embryonic fluid hyperosmotic to albumen, as has been suggested previously.