236 STRUCTURE AND LIFE HISTORIES 
216. The Result of Fertilization. (a) The immediate 
result of fertilization is physical the formation of the 
fertilization-membrane. Just how this is accomplished is 
not clearly understood. We know, of course, that the 
surface of the egg, as in every free mass of protoplasm, 
acts as a semipermeable membrane or surface, allowing 
some substances in solution, but not all, to pass through 
by osmosis. It has been suggested that, when the sperm 
enters the egg, chemical changes at once occur, which 
alter the permeability of its surface-membrane, thus per- 
mitting, for the first time, the exosmosis of some substance 
(or substances) which become transformed into the fertiliza- 
tion-membrane on contact with the sea-water. It may 
be, of course, that the substance composing the mem- 
brane is not formed until the sperm enters the egg. How- 
ever it may be caused, the formation of the membrane is 
a necessary antecedent to all subsequent changes in the 
fertilized egg; without its formation the egg dies and dis- 
integrates. 
(b) The ultimate result of fertilization, as noted in the 
preceding chapters, is biological the intermingling of the 
germ-plasms of the egg and sperm, involving the fusion 
of the two nuclei, the doubling of the chromosome number, 
and the combination, in one zygote, of the inheritances 
from two individuals. 1 
217. Artificial Fertilization. Considering that the for- 
mation of the fertilization-membrane is purely physical, 
biologists began to reason that it ought to be possible to 
induce it artificially. The experiment was first success- 
fully made by a zoologist, Loeb, with the eggs of sea-urchins 
1 The commingling of the two inheritances was called, by Weismann, 
amphimixis. 
