FERTILIZATION 



253 



gametes is brought about. In many cases it is undoubtedly merely 

 by chance: the random swimming of the sperm sooner or later 

 bringing one in contact with an egg. In other cases the move- 

 ments of the sperm seem to indicate a definite attraction by the 

 egg. Thus the sperm of some of the lower animals apparently 

 are attracted by substances eliminated by the egg at matura- 

 tion. In such instances there can be but little doubt that chemical 

 stimulation of the sperm by specific substances plays a part in 

 bringing the gametes together. This is an example of chemo- 

 tropism: a phenomenon of considerable importance, especially in 

 the behavior of free-living cells. 



B. Union of Gametes 



Once a single sperm has come into functional contact with the 

 egg, it initiates a chain of events which constitutes fertilization. 



Albumen 



Shell memb 



Chalaza 



Albumen 



Blastoderm 



Shell memb. 1 

 Shell memb. 2 



Air space 



Yolk 

 Fig. 169. — Diagram of the egg of the domestic Fowl, before incubation. 



Although, as might be expected, the variations in details are legion, 

 they do not obscure the main facts. The first reaction on the part 

 of the egg is to prevent the entrance of other sperm and thereby 

 to insure a free field for the operations of the first arrival. Fre- 

 quently a jelly-like layer is formed about the egg, or if a mem- 

 brane is already present this may be rendered impermeable or still 

 another formed. In cases where the egg is surrounded originally 

 by a dense and resistant wall, the tiny opening provided for the 

 entrance of the sperm is closed. However, the accessory wrappings 

 about certain eggs, such as those of Birds, have no relation to the 

 present subject since they are secreted, not by the egg itself, but 



