Osmotic Conditions During the Embryonic and Early Larval Life of the Brook Lamprey (Lampetra Planeri)

Abstract

1. Observations have been made on the swelling of eggs in tap and distilled water. The rate of uptake of water through the outer membranes and the surface of the ovum fell off sharply within a short time of contact with water. In distilled water uptake into the perivitelline space was very much greater than in tap water, but there were no significant differences in the volume of the ovum itself in these two media. 2. The initial permeability of the ovum to water has been estimated as 0.049 µ3/µ2atm./min., falling after about 7 hr. to 0.003 and remaining at this level for the first 5 days of development. 3. The outer egg membranes are apparently freely permeable to water and ions, and the mean Cl concentration of the perivitelline fluid in the later stages of development was found to be only 2.4 mM/l. 4. Evidence is presented which points to a decrease in the volume of the embryo at some point between gastrulation and hatching. In tap water a contraction of the whole egg was observed from the fifth day onwards. 5. The water content of the embryo at hatching was not very much greater than that of the ovarian egg. After hatching, until the absorption of the yolk, there was a continuous increase in the fresh weight and water content of the ammocoetes. 6. The evidence suggests that the ovarian egg is in osmotic equilibrium with the peritoneal fluid (mean 110 mM/NaCl/l.). During development the osmolar concentration of the total egg fluid was reduced to values similar to those calculated from the known dilution, i.e. 30-40 mM/l. At hatching, the osmolar concentration of the embryo (80 mM/l.) was raised within a few days to 11-120 mM/l. 7. The observed reduction in Cl concentration of the whole egg was much greater than that calculated from the increases in volume. The Cl concentration of the embryo itself was reduced from an initial value of 56 to 15 mM/l. embryo fluid a week before hatching. 8. Determinations of the total Cl content of whole eggs and embryos showed a very marked loss of Cl ions in the two days following fertilization. Within 35 days after hatching the Cl level had been raised by active uptake to about four times the values at the time of hatching.