214 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



in this case the diameter of the egg should be much greater. 

 This happens to be actually the case. 



If unfertilized sea-urchin eggs are placed in a weak saponin 

 solution (in sea-water) , normal membrane formation takes place 

 after a few minutes; upon leaving the eggs longer in the solution, 

 however, cytolysis ensues, and the diameter of the egg may 

 increase to double its size. 1 The same phenomenon takes place 

 if fertilized eggs are exposed to the action of saponin. If, 

 however, the contents are coagulated by heat before the eggs 

 are exposed to the saponin, or a body that has a similar effect, 

 this increase in volume no longer occurs. This appears to 

 indicate that the colloidal substance that exerts the osmotic 

 pressure is a protein. 



It may be said here that the fertilization membrane is insol- 

 uble in benzol, ether, alcohol, saponin, and similar substances. 

 Hence it is not a lipoid. 



7. Robertson 2 has extended these observations by comparing 

 the effect of different proteins on the prevention of membrane 

 formation by butyric acid. He treated the eggs of S. purpu- 

 ratus with butyric acid, but instead of putting them into normal 

 sea-water he brought them into sea-water to which various 

 quantities of different soluble proteins had been added. His 

 results are given in Table XXXVIII. 



TABLE XXXVIII 



Protein 



Highest Observed Con- 

 centration Which Per- 

 mits the Formation of 

 a Spherical Membrane 



Lowest Observed Con- 

 centration Which Pre- 

 vents the Formation of 

 a Spherical Membrane 



The mixed serum proteins 



Gelatin 



"Insoluble" serum globulin.. . . 



Casein 



Ovomucoid . 



3.70 

 1.00 

 0.30 

 0.25 

 0.125 



7.40 

 2.00 

 0.60 

 0.50 

 0.25 



1 See Figs. 39 and 45, chap. xvii. 



2 Robertson, Archiv f. Entwicklungsmechanik, XXXV, 80, 1912. 



