THE BRIDGE OF LIFE 499 



and it appears as if the cone actually pulls the sperm head and middle 

 piece into the egg leaving the tail behind. Meanwhile, the egg has 

 formed a fertilization membrane around itself, within which the early 

 stages of embryonic development will take place. This sperm head 

 which, as we have already indicated, consists almost entirely of genes, 

 then migrates inward to fuse with the nucleus of the egg and the gene 

 number and chromosome number are restored to the original figure that 

 existed before meiosis. 



Photo by Winchester 



Fig. 31.7. Human egg with its surrounding mass of follicle cells. This egg is shown 

 within a follicle of the ovary. This cut represents about a 250-diameter enlargement. 



When a sperm fertilizes an egg it brings genes in from the male 

 parent and provides biparental inheritance, with the resulting variety 

 among the offspring that would not be possible with a single parent. 

 It performs another function, however, which is equally important — it 

 provides the stimulus which is necessary to set in motion the events 

 leading to mitosis and the development of an embryo. Profound physio- 

 logical changes take place within the egg when the sperm head contacts 

 it in addition to the physical changes which we have just described. 

 For one thing, there is a tremendous increase in the rate of oxygen 

 consumption- — it will rise to about five times what it was before. This 

 provides the energy necessary for the early stages of embryonic de- 

 velopment. Also, there will be an increase in the permeability of the 

 cell membrane. This would allow a more rapid exchange of substances 



