RÉSUMÉ
Water loss was determined during incubation in 80 eggs of the domestic fowl, White Leghorn-LB and New Hampshire-NH (between the 1st and 20th day), and in 80 eggs of Beijing White ducks (between the 1st and 25th day). Our data have shown that, the lowest actual water content was found in NH eggs, the highest in the duck eggs. Water content decreased absolutely and relatively during incubation; 365 in LB, 500 in NH and 312 mg/d in ducks. Water loss was higher in the first half of hatching. In the second half it was probably partly offset by water originating from fats oxidized for energetic purposes; the offset made a third in LB and NH, and still more in the ducks. Our results are somewhat different from the literature data, the variability of which is considered to be rather high in general. There were large shifts of water among the egg components during incubation. Water totally disappeared from the albumen, although it contained approximately three quarters of water was at the beginning of hatching. Already in the first half of incubation, more than a third of water was transported to the embryo, fetal membranes and fluids. In the subsequent ten days, water accumulation in the body of the developing and growing embryo was especially rapid; the water amount increased eight times in the fowl, seven times in the duck bodies. Water losses and its shifts show not only interspecific differences related to different lengths of incubation, but also intraspecific ones, between BL and NH.