58 APPLIED BIOLOGY 



this includes all organs except the reproductive organs. The 

 latter are necessary only for the continuance of the frog 

 species; for, as we have already noted, individual animals 

 of all kinds live a relatively short time. The work of the 

 reproductive organs of the frog (and of all other organisms) 

 is the production of new individuals ; and to the general 

 facts in this line we shall now give some attention. 



57. Egg-cells and Sperm-cells. In the study of the 

 structure of the frog, we have noted the position and form 

 of the essential reproductive organs (ovaries and spermaries) 

 and their ducts by which egg-cells (from ovaries) and sperm- 

 cells (from spermaries) are conducted out of the body into 

 the water in which the animals live. From the ovaries and 

 spermaries of frogs the egg-cells and sperm-cells are dis- 

 charged in early spring. The eggs develop in the water, 

 and hatch as tadpoles. Then, after a few weeks or many, 

 depending upon the species, they develop legs, lose their 

 tails by a process of absorption, and become small frogs. 

 This change from tadpoles into frogs is called metamor- 

 phosis. 



The ovaries in a young frog are masses of tissue with numer- 

 ous small egg-cells. Each egg-cell is a spherical mass of 

 protoplasm with a nucleus near its center. As the eggs 

 grow larger, each one accumulates granules of a material 

 known as yolk; and after a time the yolk comes to occupy 

 one hemisphere of the egg, while the protoplasm is concen- 

 trated in the other. Frogs' eggs examined soon after they 

 are laid in water are seen to be black (with pigment) in one 

 hemisphere and whitish (due to yolk) in the other. The 

 black hemisphere contains most of the protoplasm. Each 

 egg is surrounded by an envelope of transparent jelly, 

 which was secreted by the cells of the oviduct as the egg 

 passed from the ovaries to the exterior. 



The sperm-cells have the form shown in Fig. 21, A. The 

 thickened part is chiefly nucleus. The tails of living sperm- 



