The osmotic behaviour of frogs eggs and young tadpoles

Abstract

Determinations of total osmotic concentration on eggs ofRana temporaria by the vapour pressure method and of chloride by theWigglesworth ultra micro-technique show a rapid fall during the first few hours, from 120 to 95 mM total concentration and, until the blastopore closes, a further slight fall to about 80 mM. Thereafter the total concentration rises rapidly, while the chloride concentration remains unaltered or (later) rises very gradually. The eggs swell considerably at first and then more gradually and the changes in concentration until the budding of the gills are due at least in the main to osmotic uptake of water. The permeability, as determined both by rate of swelling and in special experiments with heavy waterbecomes greatly reduced by fertilization. The initial permeability is calculated to correspond to a “minute Number” of 140 days, while later it is reduced to about 5 1/2 years. When the gills begin to develop permeability for water again increases and the weight rises by osmotic inflow of water. The kidneys become functional and an active uptake of salt (probably located in the gills) prevents a reduction of the osmotic concentration. The fluid in the chorionic cavity is very slightly hypertonic to the surrounding water. The degree of swelling of the egg mucus depends upon the salt concentration in the surrounding water. The swelling is enormous in distilled water and seems to be specifically inhibited by calcium.