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 cm 2 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 vital to maintaining the proper water balance. It has been suggested, in conformity with the observations of previous investigators, that the consequence of the activation process results in an alteration of the permeability characteristics of the membrane surrounding the egg proper. 6. The exchange coefficients of eggs and embryos from several teleost and amphibian species were compared at a number of similar developmental stages: it was observed that there is a general tendency for the exchange coefficient to decrease as development progresses. Although significant differences can be shown between the exchange coefficients of different species, as well as between stages within the same species, the values of E were found to occupy a rather restricted range: corrections for temperature reduced this range further so that there was little separation between many of the recorded values. The exceptions to this general ‘rule’ were the exchange coefficients of hardened eggs and embryos of Esox and Salmo , which were less than those of all other species. Yet, the depressed value of E in Esox and Salmo is not greatly different from those of amphibian egg-cells which are commonly recognized as being freely permeable to water.
1. The rate of water exchange in the salmon egg was determined by means of the electromagnetic diver-balance. 2. Salmon eggs treated with alcohol to remove the plasma membrane were used to determine the diffusion coefficient, D , for water in the cytoplasm, and untreated eggs to determine the permeability coefficient, E , for water through the plasma membrane. 3. It was found that D is of the same order as that observed in other eggs, whereas the value of E is somewhat lower, but still within the same order of magnitude. 4. The results do not confirm the classical notion that the salmon egg is impermeable to water, but they show that the exchange is extremely slow, the half time being about 24 hr. In order to explain this result the contribution of egg size and environmental temperature must be taken into account. 5. It is concluded that the actually observed rate of swelling corresponds to that anticipated from the recorded permeability coefficient, and that there is therefore no reason to presume that any kind of osmoregulation takes place.