REPRODUCTION IN PLANTS AND ANIMALS 273 



since cuttings, slips, and runners consist of many cells. Sexual 

 reproduction results in the formation of one cell which through 

 growth may become a new plant of many cells. An animal, 

 like a plant, originates from a single cell. 



299. The frog's egg. The frog or toad will serve well to 

 illustrate reproduction among animals. The frog is selected 

 for our discussion in the following sections. 



Early in the spring, when frogs may first be heard in the 

 ponds, masses of their eggs are easily found. They have the 

 appearance of small black spheres about a sixteenth of an inch 

 in diameter embedded in a transparent jelly (fig. 127). They are 

 usually found in large numbers, since a single female may lay 

 several thousand eggs. While the eggs appear black as they 

 are seen in the water, it is only the upper surfaces that are 

 dark -colored, the under surfaces being almost white. 



Each of these eggs, when laid, consists of a single cell in 

 which is stored a quantity of food material. The egg was formed 

 in the body of the female and expelled into the water when 

 mature. At the same time that the female lays the eggs, the 

 male frog expels from his body into the water a whitish fluid 

 which is shown by the microscope to contain great numbers 

 of very minute cells, known as sperms. The sperms swim 

 about actively in the water and come in contact with the 

 eggs. One of them may enter an egg and unite with it, thus 

 fertilizing it. The fertilized frog's egg resembles the spore of 

 the pond scum in that it is a single cell formed by the union 

 of two cells. Such union of cells fertilization occurs in 

 all sexual reproduction. 



300. Development of the egg. When the egg of the frog 

 has been fertilized, like the spore of the pond scum it pos- 

 sesses the power of growing into a new organism similar to 

 that which produced it. The first indication of development 

 in the frog's egg is its division into two cells. These again 

 divide, forming four cells, and the process is repeated until 

 the single large egg cell has become a mass of many small 



