PHASES OF DEVELOPMENT 379 



II. Exosmosis. 



In most cases, however, unless a second alteration is made in 

 the bathing medium, the egg will either not develop at all, or 

 will die at some intermediate stage. It is known that after 

 fertilisation, the electrical conductivity of the egg is increased 

 for fifteen minutes or so. This may be interpreted as a sign of 

 increased permeability of the membrane to surrounding salts, or 

 it may, with equal justice, be accounted for by a withdrawal of 

 water with a consequent increase in the concentration of electro- 

 lytes. During this period of increased electrical conductivity, 

 the eggs readily undergo plasmolysis if placed in a solution of cane 

 sugar. Unfertilised eggs, of course, do not show alterations at 

 this early stage in electrical conductivity nor are they easily 

 plasmolysed. This may be taken as a confirmation of the second 

 hypothesis, viz., that water is removed by exosmosis and, con- 

 sequently, the concentration of electrolytes in the egg is increased. 

 Whatever be the actual physico-chemical process brought about 

 by the entrance of the spermatozoon, the result has been imitated 

 by simple physico-chemical means. 



If, after removal from the butyric acid bath (or other cytolytic 

 agency), the egg is placed in hypertonic sea-water for about half- 

 an-hour and then returned to its normal environment, it will, 

 in all likelihood, reach maturity. That is, not only does artificial 

 membrane formation initiate the processes of development but it 

 starts, at the same time, processes which ultimately lead to the dis- 

 solution of the organism. 



These latter activities may be, for a time, suspended, by a short 

 exposure to a hypertonic solution. 



Loeb has proved that the withdrawal of water is merely the 

 trigger setting off a series of chemical as well as physical changes. 

 Attention has been repeatedly drawn in previous pages to the fact 

 that while most physical processes have a low or even a negative 

 temperature coefficient, most chemical reactions have a high 

 (Qio = 2 or more) temperature coefficient. This worker found 

 that, at a temperature of 5 C., the eggs had to remain in the 

 hypertonic solution for at least 210 minutes. The time of ex- 

 posure was decreased to 40 minutes when the temperature of the 

 solution was raised to 15. Therefore, the temperature coefficient 

 for this process is 2_i o i e Q =5 



Hence, superimposed on the physical process of exosmosis are 

 secondary chemical reactions initiated by it. 



