io Heredity and Eninronment 



with typical cell structures and functions, but with none of the 

 parts of the mature organism into which they later develop. But 

 although they do not contain any of the differentiated structures 

 and functions of the developed organism, they differ from other 

 cells in that they are capable under suitable conditions of pro- 

 ducing these structures and functions by the process of develop- 

 ment or differentiation, in the course of which the general struc- 

 tures and functions of the germ cells are converted into the spe- 

 cific structures and functions of the mature animal or plant. 



Gametes and Zygotes. — In both plants and animals the sex 

 cells are alike in many respects, though they differ greatly in ap- 

 pearances. The female sex cells of animals are called ova, the 

 male cells spermatozoa. The corresponding cells and adjacent 

 parts of flowering plants are known as ovules and pollen. Col- 

 lectively all kinds of sex cells are called gametes, and the indi- 

 vidual 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 con- 

 tains more or less food substance 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 microscopic eggs of 

 oysters, worms, etc., which may be no more than a few thou- 

 sandths of a millimeter in diameter. The human ovum (Fig. 2) 

 is microscopic in size (about 0.2 mm. in diameter) but it is not 

 smaller than is found in many other animals. It has all the char- 

 acteristic parts of any egg cell, and can not be distinguished micro- 

 scopically from the eggs of several other mammals, yet there is 

 no doubt that the ova of each species differ from those of every 

 other species, and later we shall see reasons for concluding that 

 the ova produced by each^individual are different from those pro- 

 duced by any other individual. 



The sperm, or male gamete, is among the smallest of all cells 

 and is usually many thousands of times smaller than the egg. Ira 



