236 STRUCTURE AND LIFE HISTORIES 



216. The Result of Fertilization. — (a) The immediate 

 result of fertiKzation 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.^ 



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 tlie two inlierltances was called, by Weismann, 

 amphimixis. 



