THE FERTILIZATION-REACTION 



changes that result in membrane-separation and neither 

 the consequent physical act of separation nor the changes 

 in the membrane itself. 



Nevertheless this separation of the membrane constitutes 

 an easily visible indicator of the underlying surface-changes 

 initiated by the fertilization-reaction. It tells us quickly 

 whether or not fertilization has taken place. Also, it gives 

 us information concerning the quality of the eggs fertilized. 

 Membranes that separate incompletely and are not equi- 

 distant from the egg at all points and are slow in rate of 

 separation mean eggs of poor fertilizability that subse- 

 quently develop abnormally. 



Let me emphasize that here I speak only of fertilized eggs 

 which have favorable conditions for subsequent develop- 

 ment. If after fertilization conditions are deleterious, the 

 eggs will not develop normally no matter how perfect the 

 membranes. Moreover, as we shall learn in the next chap- 

 ter, membrane-separation alone, though most perfect, does 

 not guarantee development. Indeed, even in those cases 

 in which perfect membranes are separated by experimental 

 agents, they are not wholly identical with those called forth 

 by sperm. What here looms large is the value of the 

 quality of the surface-changes for foretelling the future 

 course of the egg's development. Quite apart there- 

 fore from their value for the study of the fertilization-reac- 

 tion, the ectoplasmic changes by which the membrane 

 becomes separated have greatest significance. Develop- 

 ment embraces a series of surface-changes which vary as 

 the surface-area increases with the march of development. 

 Those occurring at the very outset are most striking; they 

 mark out the course, direct the way toward the final out- 

 come of fertilization, the formation of the complex organism 

 out of a single cell, the egg. 



205 



