EFrrect OF ForREIGN BLoop AND CELL Extracts 199 
watery extract of spermatozoa of foreign species causes mem- 
brane formation in the sea-urchin egg. The watery extract of 
the spermatozoa of the sea-urchin does not produce membrane 
formation in the sea-urchin egg. We cannot say that this is 
due to the fact that the sea-urchin sperm contains no membrane- 
forming substance. The only possible conclusion is the para- 
doxical assumption that the membrane-forming substance of 
many foreign spermatozoa can diffuse into the sea-urchin egg, 
while the membrane-forming substance of the spermatozoon of 
its own species cannot get into the egg by diffusion, but must 
be carried into it by the living spermatozoon. 
4. Experiments on the isolation of the substance present 
in the serum which is responsible for membrane formation have 
not yet met with much success. It is apparent that this sub- 
stance is comparatively resistant to heat. 
For example, ox serum was slowly heated on the water bath 
and 0.5 ¢.c. removed at different temperatures and put into 
watch glasses. On cooling, 0.5 c.c. of sea-water was added and 
a drop of eggs placed in the mixture. It was found that the 
activity of the serum was not impaired by heating it up to 
73° C. I took as criterion the number of eggs that formed 
membranes in the solution. The serum coagulated at 73° C. 
On heating the coagulum to 100°, the clear liquid that could 
be squeezed out had no longer any effect on the eggs. 
The activity of the serum is apparently not decreased by 
putrefaction; nor does extraction of the serum with ether, 
four times repeated, lessen its effect. But as ether can cause 
membrane formation and cytolysis, one must take the precau- 
tion in these experiments of not using the serum until all the 
ether has been expelled. 
If a large quantity of acetone is added to the ox serum, a 
voluminous precipitate is obtained, which hardens on drying 
into a brown crust. This substance is insoluble in sea-water. 
This dried acetone precipitate was ground up in sea-water, in a 
