8 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
those of the sea-urchin only in that either the secondary effects 
set up by membrane formation are less injurious, or that the 
eggs of these forms can recover more quickly from them than 
sea-urchin eggs. We shall see later that the eggs of different 
forms vary in this respect quantitatively, but not in principle. 
It can be shown in still another way that it is the process 
of membrane formation and not any other effect of the fatty 
acid which starts the development of the egg. For membrane 
formation induced in any other way always evokes develop- 
ment, whereas mere exposure to the fatty acid does not lead 
to development unless membrane formation also takes place. 
5. In this way, the process of membrane formation, which 
had hitherto been regarded as of quite secondary importance so 
far as development was concerned, was identified as the immedi- 
ate cause of the activation of development in the egg. The 
next step was to determine the substances and processes by 
which membrane formation might be evoked. 
I had already observed in 1904 that whenever an egg under- 
goes cytolysis it passes through a similar process of membrane 
formation, and further research along this line showed that 
indeed all cytolytic agencies cause membrane formation. A° 
relatively short exposure to a cytolytic agent leads only to 
membrane formation, whereas a longer exposure entails cytolysis. 
Hence we can say that membrane formation (and the activation 
of development) is due to a cytolysis of the surface or the cortical 
layer of the egg. Certain glucosides, such as saponin, solanin, 
and digitalin, produce strong haemolytic or cytolytic effects.- If 
eggs are exposed for a short time to a very dilute solution of 
these substances in sea-water, they form a typical fertilization 
membrane, and, if removed from the solution immediately 
alter the formation of this membrane, they will begin to develop. 
For the eggs of Polynoe this treatment is sufficient to induce 
the development to the larval stage; but the eggs of Strongylo- 
centrotus require a subsequent exposure to hypertonic sea-water 
