146 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
formation does not proceed so readily. All this harmonizes 
with the view that it is not the hydrogen ions but only the undis- 
sociated acid which diffuses into the egg and that the fatty acids 
diffuse much more quickly than the mineral acids which may 
not enter at all. 
We may inquire whether the causation of membrane forma- 
tion is a chemical or purely a physical effect of the acid. I 
asked Dr. Hagedoorn to determine the temperature coefficient 
of membrane formation by means of acids. He found that 
it is about 2 for a difference of temperature of 10° C., which 
therefore indicates a chemical reaction. The procedure was 
to measure the minimum time which eggs must remain in a 
mixture of 50 ¢.c. of acid +2.5 ¢.c. of N/10 butyric acid to cause 
membrane formation in 95 per cent of the eggs. It turned out 
that this time is twice as long at 10°C. as at 20°C. This 
result was confirmed by other determinations at different tem- 
peratures. 
All the experiments mentioned in this chapter were made on 
the eggs of the Californian sea-urchin S. purpuratus. 
