SECT. 5] IN ONTOGENESIS 805 



difference between the theoretical and calculated figures were of 

 the same sign, i.e. the eggs always contained slightly less water 

 than on the complete impermeability hypothesis they should have 

 done. Faure-Fremiet explained this by suggesting that a very small 

 degree of permeability to electrolytes was present. Data for urea 

 solutions gave the same results, and are shown plotted in the same 

 figure. Fig. 191 shows similar experiments done with neutral salts, 

 such as sodium chloride, magnesium chloride and calcium chloride. 

 Here, however, at the higher osmotic pressures the calculated and 

 observed values diverge to a significant extent, which indicates that 

 the absolute impermeability of the membrane is not maintained 

 under such conditions. Probably electrolytes then enter the cell, 

 just as in sugar solutions they may to a slight extent come out of 

 it. These effects were so slight, however, that Faure-Fremiet was 

 inclined to see in them an adsorption of ions on the external 

 surfaces of the egg-membrane rather than a true permeability. Acids 

 and bases did not seem to penetrate at all into the eggs oiSabellaria. 

 Temperature had a slight effect on the imbibition of water by the 

 eggs; outside a constant range between 18° and 25°, the imbibition 

 increased at low and decreased at higher temperatures. The margins 

 of temperature between which the imbibition is at its normal value 

 corresponded exactly to those between which perfectly normal de- 

 velopment is possible. These effects have obviously an important 

 bearing on the problem of egg viscosity, which has been handled 

 (/'by so many workers. Again, there was a modification of imbibition 

 by the eggs according to change in pH — at pYi 5 they contained 

 sHghtly less water than normal, at pH 7, 6 per cent, more (/?H 8-4 

 normal), and at j^H 12, 10 per cent. more. These small differences 

 were perhaps related, according to Faure-Fremiet, to the isoelectric 

 point of the egg-proteins. The effects of these various agents on 

 the imbibition, the water-content, and therefore the osmotic pressure 

 of the egg-contents, were of the following comparative magnitudes : 



Maximum °o 

 variation 

 External osmotic pressure ... ... i8o 



Specific action of cations ... ... 70 



Temperature ... ... ... ... 14 



P^ 7 



The heat factor was subsequently examined in more detail by 

 Ephrussi, using the eggs of sea-urchins. He obtained an exactly 



