SECTION 5 



BIOPHYSICAL PHENOMENA IN ONTOGENESIS 



5-1. The Osmotic Pressure of Amphibian Eggs 



A considerable number of workers have turned their attention to 

 measurements of osmotic pressure in eggs and embryos, in the attempt 

 to throw some Hght either, in the case of aquatic embryos, on the 

 relation between the embryo and its environment, or, in the case 

 of birds, for instance, on the relations as regards water and salts, 

 between the different components of the ovum. 



The best known part of the work has been done on the eggs of 

 amphibia. The fundamental observation was made by Backmann & 

 Runnstrom in 1909 that the osmotic pressure of frog's egg-Breis 

 was very different according to the stage of their development. They 

 obtained the following figures : 



A (depression of 

 the freezing-point) 

 (°C.) 

 Ripe ovarial eggs ... ... -0-48 



Fertilised eggs ... ... —0-045 



Embryos of 5 days ... -0-23 



Tadpoles of 20 days ... —0-405 



Serum of adult ... ... -0-465 



From these simple observations, which have often been confirmed, 

 all the subsequent work took its origin. They were in many ways 

 interesting, for the A of ordinary pool fresh water was found to be 

 — o-o6, so that after the eggs were shed from the oviduct they 

 evidently adjusted their osmotic pressure to equal that of their im- 

 mediate environment. Then, later, they acquired some kind of inde- 

 pendence, and by the time of hatching were well on their way to 

 attaining the adult osmotic pressure. The question as to how the 

 initial fall in osmotic pressure was achieved was not easy to answer. 

 It might be accounted for, of course, on the supposition that the 

 eggs took up water from their hypotonic medium, and diluted their 

 contents, but if this had been so the increase in volume would have 

 been far greater than was actually found. Backmann & Runnstrom 

 preferred to assume that the egg-colloids were rapidly gelated 

 after the eggs were shed, and inorganic ions adsorbed on to them, a 

 process which would certainly lower the osmotic pressure, but would 



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