38 CLEAVAGE II 



some magnesium chloride had been added. The superiority 

 of the results obtained by the use of this salt led Loeb 

 at first to attribute the effect to some specific activity of the 

 magnesium ion. 



Since however the solution used was hypertonic to sea- 

 water, the question arose whether the determining cause 

 might not be the increased osmotic pressure. Experiments 

 were therefore instituted in which the eggs were subjected 

 temporarily to the action of solutions of a number of sub- 

 stances in sea-water, the solutions being all hypertonic to 

 sea-water but all isotonic with one another. The substances 

 employed were not only salts like magnesium chloride, sodium 

 chloride, and potassium chloride, but also non-electrolytes 

 such as cane-sugar and urea. After temporary immersion in 

 any of these solutions the eggs segmented and developed. 

 The hypothesis was then adopted that the increased osmotic 

 pressure, resulting in a withdrawal of water from the eggs, 

 was responsible for the result. The hypertonic solutions must 

 be alkaline and contain oxygen. 



It is to be noticed that development was not perfect in any 

 of these cases, for the fertilization membrane was not formed, 

 the cleavage was very irregular simultaneous division into 

 several cells, and multipolar mitoses occurred and the larvae 

 sank to the bottom instead of swimming at the surface. 



Further investigation showed however that solutions which 

 were physically isotonic did not produce precisely the same 

 effect, were not in fact physiologically equivalent, and con- 

 versely that solutions which had the same physiological effect 

 had not the same osmotic pressure, as may be seen from the 

 accompanying table (Table IV). 



TABLE IV. 



Optimal concentration Osmotic pressure 



Solution. in gram-molecules. Dissociation, in atmospheres. 



Cane Sugar 0-96 m. 21-53 



Grape Sugar 1-04 m. 23-33 



CaCl 2 0-50 m. 64% 25-57 



MgCl 2 049m. 70% 26-47 



LiCl 0-74 m. 66 % 27-59 



NaCl 0-79 m. 71 % 30-28 



KC1 0-78 m. 77 % 30-95 



