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.'| 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 osmotie 
pressure is a protein. 
It may be said here that the fertilization membrane is insol- 
uble in benzol, ether, alcohoi, saponin, and similar substances. 
Hence it is not a lipoid. 
7. Robertson’ 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 XX XVIII. 
TABLE XXXVIII 
Highest Observed Con-| Lowest Observed Con- 
Protein centration Which Per-|centration Which Pre- 
mits the Formation of, vents the Formation of 
a Spherical Membrane|a Spherical Membrane 
The mixed serum proteins..... 3.70 7.40 
Gelatin seek ee ee 1.00 2.00 
“Tnsoluble”’ serum globulin. . 0.30 0.60 
Caseinetiee nosis cee eens 0.25 0.50 
Ovomucoidie na enero 0.125 0.25 
1 See Figs. 39 and 45, chap. xvii. 
2 Robertson, Archiv f. Entwicklungsmechanik, XXXV, 80, 1912. 
