| Summary: | 1. The amount and the rate of exchangeable water was determined in cell analogues and normally developing egg-embryos of the pike within the developmental interval from egg shedding to advanced myomere embryos using the automatic diver-balance. 2. Comparisons between the volume of water exchanged and the percentage water content obtained by wet-dry weight determinations confirmed the view that essentially the entire water content of the whole egg - chemically treated and untreated - was exchanged with the isotopic medium. When developing specimens were placed in isotope-free media at the conclusion of the water exchange experiment, they readily exchanged the accumulated isotope for ordinary water and continued to develop normally. On the basis of the foregoing observations, it was concluded that, during the embryological period under investigation, the developing organism undergoes a continuous cyclic water turnover with the environment. 3. Water exchange in chemically treated eggs (= cell analogues) proceeded rapidly, in one continuous uninterrupted phase indicative of a diffusion process in the absence of a surface barrier. The diffusion coefficient, uncorrected for the possible affect of unstirred layers, was about the same as Salmo salar : 6 x 10-6 cm2 sec-1, at 9·0°C. 4. The pattern of water exchange in untreated eggs was distinctly different than that of the treated specimens, proceeding in a two-step manner: (1) a rapid, initial exchange of the fluid-filled perivitelline compartment (2) followed by a prolonged exchange of the egg proper which was characteristic of a diffusion process in the presence of a surface restriction to water flow. The exchange coefficient of unhardened eggs, immersed in Ringer solution to inhibit chorionic hardening, was considerably higher (1·8 x 10-5 cm sec-1) than the hardened specimens (2-4 x 10-6 cm sec-1). 5. Additional observations of the affect of Ringer solution upon egg swelling and the exchange coefficient strongly support the view that the total activation process is ...
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