218 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
Another hypothetical suggestion is that the fertilization 
membrane is preformed in the unfertilized egg and is merely 
the peripheral film of protoplasm which is lifted up from 
the egg through the swelling and liquefaction of some protein 
lying underneath in the cortical layer of the egg. When lifted 
up from the egg the preformed membrane undergoes a modifica- 
tion; it becomes thicker and tougher. The objection has been 
raised that no such surface film is visible in the unfertilized egg, 
but this objection is not valid, since the surface films which 
form on the principle of Thomson-Gibbs are beneath the limit 
of visibility. This fact also meets the objection of Moore,} 
that after shaking unfertilized eggs into fragments, each frag- 
ment can still form a fertilization membrane upon the entrance 
of a spermatozoon, since each fragment is bound to have at its 
surface such a film. But we are now in the realm of mere 
hypotheses to which it is not worth while to devote much space. 
1 Moore, University of California Publications, Physiology, LV, 89, 1912. 
