498 



THE BRIDGE OF LIFE 



Under normal conditions only one sperm enters the egg. One 

 sperm is needed to restore the normal number of chromosomes for the 

 body cells, whereas more than one would completely upset the chromo- 

 some balance. As soon as one sperm comes in close contact with the 

 surface of the egg, something happens which repels other sperms. Some 

 chain reaction must travel over the surface of the egg which causes it 

 to reject all other sperms which may chance to come in contact with 



Photos by Winchester 



Fig. 31.6. Living human sperms. The upper photomicrograph shows a flat view of 



the head, and a side view of the head is shown in the lower photomicrograph. These 



pictures were made with a phase contrast microscope. The sperms as they appear in 



this cut are enlarged about 2,000 diameters. 



it, and those already held there are now released from the trap. It 

 must be a very rapid reaction, or else two or more sperms would es- 

 tablish intimate contact and start to enter the egg before the repulsion 

 was complete. 



In the next phase of fertilization the egg bulges out at the point 

 where the sperm is in contact with it and forms what is known as a 

 fertilization cone. The sperm head becomes embedded in this cone. 



