A speim 



ceU 



An egg cell 



Sperm entering egg All ckromosomes distinct 



Nuclei fuse; 



chromosomes mixed 



at random 



Cliromosomes 

 form tangle 



Chromosomes 

 distinct,each 

 splitting lengthwise 



Chromosome 

 halves separate 



THE CHROMOSOMES IN FERTILIZATION 



The essential fact about fertilization, in both plants and animals, is the uniting of 

 chromosomes from two cells into one nucleus. Although the male and female gametes 

 are quite different in most common species, the chromosomes supplied to the zygote 

 by the two parents are almost identical 



In most ancient ci\'ilizations people believed that "fruitfulness", or the 

 producing of offspring, in many plants, as well as in most animals, depends 

 upon two parents, male and female. They recognized and accepted the fact 

 that the members of most species exist in two forms, male and female. "Male 

 and female created he them." The oldest myths and legends make a point of 

 sex differences. But the exact connection between sex and reproduction could 

 not be known until the microscope had been invented and improved. 



It was as recently as 1875 that a German physician and embryologist, 

 Oskar Hertwig (1849-1922), was able to show that the essential fact in fertiliz- 

 ing, or "making fruitful", is a uniting of two di^erent cell nuclei into one (see 

 illustration above). 



While the two gametes are indistinguishable in some species, the combining 

 cells in most plant and animal forms differ from one another in many ways. And 

 in most species of animals the two kinds of germ cells, or gametes, are borne by 

 the two different kinds of individuals, male and female. The female gamete is 

 the egg, and the male gamete the sperm (see illustration, p. 388). The union 

 of a sperm with an egg, or fertilization, takes place in all sexual reproduction. 



376 



