102 ELEMENTARY ZOOLOGY. 



when there has been a union of the two cells into one 

 (Fig. 19, c). In this union there is a fusion of the male 

 nucleus and the female nucleus into one nucleus, which 

 occupies the protoplasm of the ovum. This result is 

 called a fertilized ovum or an embryo. . 



This new organism, produced by the fusion of the two 

 different kinds of cells, has a renewal of growth-power 

 much greater than that possessed by either alone. It 

 may develop muscle and bone and connective tissue and 

 nerve cells. Fertilization has apparently made the egg- 

 cell young again. It enters upon its youthful stages of 

 development and has none of the age of the parent. 



1 08. Other Modes of Renewal of Youth. There are 

 in nature certain other methods of renewing the youth 

 of animals that have long been active. Such animals, 

 usually unicellular or the lower sort of many-celled ani- 

 mals, may surround themselves with a thick wall and 

 cease entirely from their activity. After a period of 

 rest, the cyst may break and the protoplasm comes forth 

 apparently with renewed power to multiply and grow. 



In a good many animals, as plant-lice, bees, and 

 others, it often happens that certain ova may develop 

 without the union of the male cell. This is known as 

 parthenogenesis. We do not know what gives them 

 this power. 



In recent years it has been shown that many kinds of 

 eggs may be artificially fertilized by being placed in water 

 containing materials in solution somewhat different from 

 those to which the eggs are accustomed. This means 

 that in some way this renewal of the youthful power of 

 development is brought about by the stimulus applied 

 from without apparently chemical. 



