268 



SEXUAL REPRODUCTION 



The photomicrograph of a mold culture 

 which shows several zygospores. 



by keeping proper food present, thousands of generations have 

 been produced from a single healthy individual of a certain species. 



Under these conditions, conjuga- 

 tion has not taken place between 

 anv two animals, but certain in- 

 ternal changes occurred which 

 were observed and described. 



Sexual reproduction in higher 

 plants and animals. In the sexual 

 method of reproduction of the 

 Spirogyra, the entire cell acts as 

 the gamete. In the Paramecium, 

 simply a part of the micronucleus 

 acts as the gamete, but like-sized 

 portions of the micronucleus unite. When gametes are similar they 

 are known as isogametes or like gametes. In most higher plants 

 and animals, certain specialized organs produce cells that form 

 gametes. There are two kinds of gametes. One, a very small 

 cell usually equipped with a motile tail, is called the microgam- 

 ete or the sperm cell. ^ The other type of reproductive cell is the 

 receiving or passive cell. It is larger and is 

 called the macrogamete or egg. It usually 

 stores food particles called yolk granules. 

 When the egg is ripe, a sperm may penetrate 

 the egg. The nucleus of the sperm fuses 

 with the nucleus of the egg. This process 

 is called fertilization. The cell formed by 

 the union of the egg and sperm cell is 

 known as a zygote. 



Maturation. Every species of an organ- 

 ism has a constant number of chromosomes. 

 The number is always the same for that species. During fertili- 

 zation, when two cells unite, the number of chromosomes would 



Photomicrograph of filaments 

 of Spirogyra showing conjuga- 

 tion and zygospores. 



