FACTS AND FACTORS OF DEVELOPMENT 525 



pinging energies, or stimuli, in a manner which is usually, but not 

 invariably, adaptive or useful. 



Both the egg and the sperm are living cells with typical cell structures 

 and functions, but with none of the parts of the mature organism into 

 which they may 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 producing these structures and functions by the process of develop- 

 ment 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 appearances. The female sex cells of 

 flowering plants are called ovules, the male cells pollen. The corres- 

 ponding cells of animals 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 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 miscroscopic eggs of oysters and worms, which may 

 be no more than a few thousandths of a millimeter in diameter. The 

 human ovum 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 

 characteristic 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 produced 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. In most ani- 

 mals, and in all vertebrates, it is an elongated, thread-like cell with an 

 enlarged head which contains the nucleus, a smaller middle-piece, and a 

 . very long and slender tail or flagellum, by the lashing of which the 

 spermatozoon swims forward in the jerking fashion characteristic of 

 many monads or flagellated protozoa. In different species of animals 

 the spermatozoa often differ in size and appearance, and there is every 

 reason to believe that the spermatozoa of each species are peculiar in 

 certain respects even though we may not be able to distinguish any 

 structural differences under the microscope. The human spermatozoa 

 closely resemble those of other primates but are still slightly different, 

 and the conclusion is inevitable, as we shall see later, that the sperm a- 







