LIVING MATTER 133 



have different numbers, which sometimes reach more than a 

 hundred in every cell. Each cell in the body tissues of a man 

 contains forty-eight chromosomes. 



Protoplasm and Life. Though protoplasm is made up of 

 well-known substances it can be produced only by proto- 

 plasm. Many scientists have tried to mix materials to- 

 gether to produce protoplasm. Although they can produce 

 something which looks like protoplasm, no experiment has 

 ever produced living protoplasm ; in fact, it is very doubt- 

 ful if living protoplasm can ever be formed artificially. 



Mitosis during Sexual Reproduction. In the chapter deal- 

 ing with life processes mention was made of the fact that 

 reproduction is usually through specialized products called 

 germ cells. In the formation of the young from the germ 

 cells we find one of the most conspicuous instances of cell 

 division. The changes which lead up to the multiplication 

 of cells in the embryo will be described here. 



Germ Cells. Germ cells produced by females are rela- 

 tively large cells containing stored yolk to be used as 

 food material by the developing young (Fig. 72, A). The 

 cells produced by the males are very small cells (Fig. 72, B, 

 sperm cell), capable of locomotion, called spermatozoa or 

 sperm. Neither of these minute bodies in any measure re- 

 sembles the animal which produced it. Yet after a long 

 series of steps, or stages, an egg and a sperm unite and finally 

 form an individual like the animals which produced them. 



Maturation and Fertilization. The act whereby an egg and 

 a sperm are united to start the formation of a new individual 

 is called fertilization (Fig. 72, B-D). Before this act can 

 take place both egg and sperm must have gone through a 

 period of preparation, which is called maturation. During 

 maturation the most important changes take place in the 

 nuclei of the germ cells. It will be remembered (p. 131) 

 that the nucleus contains a substance called chromatin, and 



