PHYSICAL CHANGES IN THE CONSTITUENT PARTS OF DEVELOPING SALMON EGGS

Abstract

Wet weight and dry weight determinations were made on salmon eggs, egg capsules, detached embryos, and yolk sacs from fertilization to the end of yolk absorption. One capsule weighs 5.0 mg. wet and 2.3 mg. dry. No significant change could be found during development. Water hardening of the eggs, which occurs on transfer from coelomic fluid to water, is independent of fertilization. Increased tension of the capsule (resistance to breakage) becomes noticeable two hours after transfer to water and proceeds at a maximum rate for 30 hr. By contrast the uptake of water begins immediately on transfer from coelomic fluid and is virtually completed within an hour. It is concluded that the stoppage of water uptake is not due to the hardening of the capsule. The theory is presented that (a) all the water taken up becomes perivitelline fluid, (b) there is no loss of salt or gain of water by the yolk, (c) the transfer of water through the capsule is not due to osmosis but to imbibition by protein liberated into the perivitelline space by the yolk. The mortality rate for eggs reaches a maximum 14 days after fertilization (i.e., 36 days before hatching). The water content of the yolk decreases steadily during development, from 63.5 to 55.5%. Embryos from small eggs tend to compensate by growing more rapidly than those from large eggs, through the mechanism of extra water uptake. As to efficiency, which proved independent of egg size, the embryo wastes 30% of the yolk in turning the other 70% into living tissue.