PHYSIOLOGICAL CHANGES AT FERTILIZATION 47 



Morphological changes at fertilization 



Once the sperm reach the surface of the egg a number of visible changes 

 occur; these lead to the entrance of one sperm and the fusion of a sperm and 

 an egg nucleus. The first thing that occurs is a negative reaction of the 

 surface of the egg toward other sperm. There are usually thousands of sperm 

 for each egg, and the individual sperm is small. As we picture, when it comes 

 in close contact with the egg at any point (Fig. 21), a reaction travels over 

 the surface of the egg so that a second sperm is rejected. Only one sperm, 

 therefore, can, under normal conditions, enter the egg. Under experimental 

 conditions several sperm may enter and abnormal development results. There 

 must be a very rapid reaction over the surface of the egg, because with large 

 numbers of sperm two may reach the surface almost simultaneously. Thus, 

 we assume a very rapid chain reaction, which starts from the point of attach- 

 ment of the first sperm and spreads rapidly from this point. 



After this negative reaction a cone of protoplasm develops at the point 

 of contact of a sperm. The egg bulges out, and this extrusion is called the 

 fertilization cone. The sperm becomes embedded in this cone of protoplasm. 

 The fertilization cone appears to pull the sperm into the egg. How it does 

 this is not clear, but shortly after the fertilization cone forms it retracts to 

 the surface and then the sperm is inside the egg, in the protoplasm itself. 

 The sperm head, which is almost completely nucleus, then undergoes changes, 

 and the sperm nucleus and the egg nucleus fuse. The chromosomes appear as 

 distinct bodies and the first cell division takes place. 



Physiological changes at fertilization 



But accompanying these visible phenomena which have just been 

 described, some deep-seated physical and physiological changes occur in the 

 protoplasm of the egg. At about the time that the negative reaction takes 

 place — that is, even before the sperm is inside the egg — we can detect 

 a tremendous increase in the rate of oxygen consumption of the egg. The 

 increase finally reaches about 500 per cent. Along with the increased intake 

 of oxygen we can measure an increase in the heat production. Both these 

 changes indicate that the rate of chemical reactions has been considerably 

 increased by the stimulation due to the sperm contact. This increase would 



