THE BRIDGE OF LIFE 497 



than it would be if the divisions had been equal. Of course, only one 

 of the four cells can become an egg, but this is no disadvantage because 

 the number of eggs produced is small in comparison with the number 

 of sperms. In a woman, for instance, there will be only about 350 eggs 

 released from her ovaries during her entire lifetime. Each of these 

 eggs has a much greater chance of being fertilized than a sperm has of 

 fertilizing an egg, so the smaller number of eggs is sufficient. Even in 

 those animals where the number of eggs produced by the females is 

 very large the males still produce millions of times as many sperms. 



Fertilization 



Now that we have learned how the gametes are produced we will 

 turn our attention to the method by means of which the sperms and eggs 

 come together to form zygotes which will grow into new individuals. 

 In many water animals this problem is solved simply — the eggs and 

 sperms are released in the water and the sperms swim to the eggs and 

 fertilize them. In land animals, however, there must be a method of 

 transferring the sperms directly to the bodies of the females by the males 

 by the process known as copulation. Plants, with their restricted 

 motilitv, have developed pollen grains which can be carried by the wind 

 or insects and thus bring the male gamete to the female gamete, but 

 such a method is not found among animals. Whether released in the 

 water or in the body of the female, however, the sperms still must 

 travel under their own power in the final stages of the journey to the 



eggs. 



The meeting of the egg and sperm is believed to be due to chance. 

 When millions of sperms are deposited near an egg, they swim in all 

 directions, and some are certain to come into contact with it almost im- 

 mediately. In a short time they form a thick mass, each sperm with 

 its head pointed inward as if it is trying to penetrate the membrane 

 surrounding the egg. The egg is known to contain a substance which 

 causes the sperms to stick to its surface — they are trapped. It was 

 formerly believed that the egg secreted a chemical which diffused 

 through the water and caused the sperms to swim directly to it, but 

 recent studies have failed to confirm this theory. 



group and the twelve, including the X-chromosome, in the upper group. In the prophase 

 of the second division of meiosis (F) the double nature of the chromosomes is very 

 evident as the chromatids are clearly separated. In the anaphase (G) the chromatids 

 have pulled apart and now form separate chromosomes which appear as single rods. 

 In the telophase (H) the secondary spermatocyte has almost completed division into 

 two spermatids. Finally, the spermatids (I) are shown undergoing transformation 



into sperms. 



