786 BIOPHYSICAL PHENOMENA [pt. in 



fertilised ones was only 1-47 mm. These relations confirmed Back- 

 mann's results; Backmann, indeed, had found that the unfertilised 

 salamander egg burst in 2 to 7 hours, if placed in water, but not if 

 placed in salt solution. McClendon observed that, if the fertilised 

 eggs were placed in salt solutions, development often ceased at the 

 gastrula stage, and he concluded that the exit of salts from the egg 

 was a physiological necessity. 



Bialascewicz's views on the importance of the perivitelline fluid 

 were accepted by McClendon, who analysed it in the cases o^ Ambly- 

 stoma and Cryptorhyncus , and found salts and organic substances, but 

 only traces of protein (o-i6 per cent, dissolved solids). Bialascewicz's 

 finding, that immediately after fertilisation there was a measurable 

 decrease in the diameter of the egg-cell, evidently reflected the forma- 

 tion of the perivitelline fluid. Backmann & Runnstrom had suggested 

 that the fluid might be secreted by the suckers of the embryo, but 

 the case of the salamander, which has no suckers, is contrary to this 

 view. The normal course of events in the amphibian egg, therefore, 

 after laying, is as follows : if fertilisation occurs, the osmotic pressure 

 of the egg drops sharply to a low value, perhaps because of the intra- 

 cellular fixation of osmotically active substances, but more probably 

 because some of these are excreted into the perivitelline fluid, and to 

 a certain extent into the surrounding medium. Then there occurs 

 a gradual rise, leading back to the initial value, probably brought 

 about by the increase of osmotically active substances produced in 

 metabolism, and not by any absorption of salts from without. The 

 skin of the adult frog absorbs water at a rapid rate, and if it were 

 not for the action of the kidney the animal would die of oedema. The 

 pronephros develops and is believed to be functional well before 

 hatching, so the frog embryo and larva is well protected against 

 this possibility. 



5'2. The Genesis of Volume-regulation 



More recently the problem of the origin of water regulation in 

 amphibia has been investigated by Adolph, who studied the change 

 in volume or weight when different developmental stages of Rana 

 pipiens were placed in various solutions. The unfertilised eggs some- 

 times swelled, and sometimes shrank, but in none of the experiments 

 did the change in volume follow exactly the concentration of the 

 environmental salt solution. This was in contradiction with Back- 



