790 



BIOPHYSICAL PHENOMENA 



[PT, III 



5-3. The Osmotic Pressure of Aquatic Arthropod Eggs 



The osmotic behaviour of the amphibian egg is to some extent 

 paralleled by that of the eggs of cladocerans, on which Przylecki 

 has made notable studies. He found that the youngest embryos 

 in his series had the lowest osmotic pressures; thus at 6 hours it 

 was A — 0-245° for Simocephalus vetulus, and — 0-186° for Daphnia 

 magna. As development pro- 



ceeded, the pressure steadily 

 rose, reaching at 54 hours 

 — 0-752° in the former case, 

 and at 84 hours — 0-739° in the 

 latter. The curve is shown in 

 Fig. 183 taken from Przylecki's 

 paper. The eggs were par- 

 thenogenetically " fertilised," 

 and the osmotic pressure was 

 not determined by direct freez- 

 ing-point measurements, but 

 by observing how strong a 

 v'' glucose solution was required 



A(°) 



o Simocephalus vetulus 



• Daphnia pulex(embryo) 



X " " (perivitelline liq.) 



© Daphnia magna(parthenogenetic) 



B " " (ordinary fertilsation) 



+ " " (perivitelline liq.) 



Fig. 



to make no change in ^gg- or embryo-volume^. Przylecki regarded the 

 rise in osmotic pressure as largely due to the formation of osmotically 

 active substances in metaboUsm. At the 6th hour the egg-membrane 

 is in a state of tension, which augments until the 24th hour ( 1 20 per 

 cent.), but then ceases altogether up to the 60th hour, after which it 

 rises again (to 167 per cent.), as is shown in Fig. 184. In spite of the 

 mounting osmotic pressure, then, there is a period during which no 

 increase in membrane tension takes place'^. At this time the membrane 

 does not return to its original state if the pressure is relieved by 

 pricking the Qgg, but has evidently expanded in a more plastic 

 manner. At the 60th hour the outer membrane (egg-membrane) 

 bursts, and the larval membrane begins to expand. The growth of 

 the embryo in volume follows these limiting factors. 



In a second paper, Przylecki confirmed the earlier results, working 



^ And assuming that the permeability of the egg-membranes for glucose was the 

 same as that for salts. 



^ Strictly speaking, this is not tension, but rather the amount by which the elastic 

 limit is exceeded. 



