366 ELEMENTS OF GENERAL SCIENCE 



season. At some later time the resting cell grows again, 

 producing a new plant like those that formed it. 



374. Animals and plants begin as one cell. The resting 

 cell is called a spore, which means a special reproductive 

 cell which can form a new plant like those that formed the 

 spore. In forming the spore the mass of protoplasm in one 

 cell is fertilized by having the protoplasm from the other 

 cell unite with it. Reproduction that is brought about by 

 having two cells unite to form the reproductive cell is called 

 sexual reproduction. The spore thus formed may be called 

 the sex cell or sex spore. In pond scum many paired cells 

 of two adjacent plants may produce sex spores, and many 

 new plants may thus be provided for. 



This one sex cell begins the growth of a new pond-scum 

 plant, and by successive divisions of the cells thus formed a 

 many-celled plant may soon be grown. Whether a plant or 

 animal in its adult form consists of one or of many cells, it 

 consists of one cell at first. This is not true usually of plants 

 or animals formed vegetatively, since cuttings, slips, and run- 

 ners 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 



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

 illustrate reproduction among animals. 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, em- 

 bedded in a transparent jelly (fig. 170). They are usually 

 found in large numbers, since a single female may lay sev- 

 eral 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 



