REPRODUCTION 419 



and provides a store of food material for the first part of the future 

 organism's life, while the male cell or spermatozoon is the smaller and 

 has organs of motion which provide the means of moving through the 

 usually short distance that separates the two when they are deposited 

 for union, the female cell being passive. 



When united, the ovum and the spermatozoon form a single cell, the 

 oosperm or zygote, of at least equal value quantitatively and qualitatively 

 to any cell in either of the parent organisms. This oosperm must be 

 looked upon as the ideal cell, representative of its species, and it is by 

 its subsequent divisions and the differentiation of most of its descendants, 

 the somatic cells, that the new organism is formed. 



Some of its descendants, however, do not differentiate but divide each 

 time into cells exactly homologous to the oosperm (Fig. 379).^ These 

 are the reproductive cells or germ cells of the organism and they form, 

 as it can now be seen, an unbroken series from one generation to the 

 next. They become smaller by dividing, but they divide nearly all their 

 parts equally and are always the same. They become ready in the 

 adult organism to grow in size, to mature, and to again part with their 

 parent body, which they leave to die while they unite with another repro- 

 ductive cell to form a new organism. Until the growth and maturation 

 period arrives, however, the cells can be distinguished neither as male 

 nor female, although their sex is probably already determined. They 

 lie in more or less compact masses in various parts of the bodies of 

 different animals, and form, together with connective tissue and other 

 tissues, organs called the gonads. 



The gonad may be a collection of tissues which develop de novo each 

 breeding season to hold the reproductive cells or it may be a very per- 

 manent organ. It usually has associated with it a great complexity of 

 accessory tissues and organs (genitalia) intended to facilitate the union 

 of the spermatozoa with the ova and to further aid them in their repro- 

 ductive career (brood pouches). A gonad containing male cells is 

 known as a testis and one containing female cells as an ovary. Both 

 male and female reproductive cells may occur in the same gonad or in 

 different gonads in the same individual, in which case the species is 

 known as a monoecious one. When the male and female cells are found 

 in separate individuals it is known as a dioecious species. Both cases are 

 common. The higher and more specialized kinds of animals are usually 

 dioecious. The monoecious forms seldom unite an ovum and a sperma- 

 tozoon from a single individual to form an offspring. An exchange of 

 sperm between two individuals is the usual method. 



The reproductive cells are the important or specific cells of a gonad, 

 while there are several other kinds of cells that play secondary but 

 necessary parts in its structure. Certain cells that feed the young 



