PACTS AND FACTORS OF DEVELOPMENT 13 



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 fundamentally alike, though they differ 

 greatly in appearance. The female sex cells 

 of flowering plants are called ovules, the male 

 cells pollen. The corresponding cells of ani- 

 mals are known as ova and spermatozoa. Col- 

 lectively all kinds of sex cells are called 

 gametes, and the individual formed by the 

 union of a male and 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 form 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 



