270 SEXUAL REPRODUCTION 



production of the mature eggs is called oogenesis. All of the 

 spermatozoons or sperms of a species are of the same size and 

 structure. All may function. Each of them has a tail-like 

 structure of cytoplasm called the flagellum. By means of the fla- 



gellum the sperms are 

 able to move about. 



_ Oogenesis is essentially 



^^% the same as spermato- 



Many sperms of animals may swim about the mature egg. o-pnpm'^ Onp Inro-p ppll 



One sperm penetrates the cell membrane of the egg and & eij Algei 



carries, in this case, four chromosomes. These are added jg |^ e real functioning 

 to the reduced number in the nucleus of the mature egg. 



The union of sperm and egg with the restoring of the original gamete, Called the ma- 

 number of eight chromosomes is fertilization. . 



ture egg. It contains 



the food supply or yolk. In the process of maturation of the egg, 

 three minute cells known as polar bodies are thrown off. 



Fertilization. After the maturation process, the gametes 

 formed from the primary sex cells are ready for union or fertiliza- 

 tion. The sperm is always in a moist environment and by means 

 of its flagellum swims to the egg. In some eggs there is a small 

 opening in the membrane called the micropyle. Through this 

 opening one sperm enters. When an egg lacks the micropyle, a 

 sperm penetrates the plasma membrane. The head of the sperm 

 contains the nucleus. The flagellum or tail of the sperm is usually 

 left outside of the egg. After a sperm nucleus enters the egg, a 

 chemical change takes place within the cell membrane. This 

 permanently seals the egg so that no more sperms can enter. 

 The nucleus of the egg and of the sperm fuse. The entrance of 

 the sperm restores the original number of chromosomes in the 

 egg cell. Mitosis of the fertilized egg then takes place. It differs 

 from typical mitosis in that half the number of chromosomes 

 came from the sperm and half came from the egg. The chromo- 

 somes are said to be paternal and maternal in origin. Immediately, 

 mitotic division or cleavage takes place and two cells are formed, 

 each with the same number of chromosomes, as the species 



