FACTS AND FACTORS OF DEVELOPMENT IS 



producing these structures and functions by 

 the process of development or differentiation, 

 in the course of which the general structures 

 and functions of the germ cells are converted 

 into the specific structures and functions of the 

 mature animal or plant. 



In both plants and animals the sex cells 

 are alike in many respects, though they differ 

 greatly in appearances. The female sex cells 

 of animals are called ova, the male cells sperm- 

 atozoa. The corresponding cells and adjacent 

 parts of flowering plants are known as ovules 

 and pollen. Collectively all kinds of sex cells 

 are called gametes, and the individual formed 

 by the union of a male and a female gamete is 

 known as a zygote, while the cell formed by the 

 union of egg and sperm is frequently called 

 the oosperm. 



The egg cell of animals is usually spherical 

 in shape and contains more or less food sub- 

 stance in the form of yolk; it varies greatly in 

 size, depending chiefly upon the quantity of 

 yolk, from the great egg of a bird, in which 

 the yolk or egg proper may be hundreds of 

 millimeters in diameter, to the miscroscopic 



