CHAPTER XXVI 

 REPRODUCTION IN ANIMALS 



Reproduction. — Each species of living thing persists 

 from century to century, while the life of the individuals 

 composing it is of comparatively brief duration. A dog 

 or rabbit, for example, comes into existence, grows to a 

 definite size, lives its allotted time, and provides for the 

 continuation of the race by producing offspring to take 

 its place. This process of producing new individuals, and 

 thus insuring the perpetuation of the species, is known 

 as reproduction. 



In the many-celled animals the division of labor among 

 the cells composing the body is associated with the de- 

 velopment of male and female reproductive or germ- 

 cells, which under certain conditions to be described 

 later, develop into new organisms. This is. known as 

 sexual reproduction. In a host of lower animals the 

 body divides to form two offspring; or bud-like out- 

 growths are formed which separate from the body and 

 become new individuals. Sex cells, in such cases, play 

 no part whatever, and hence this type of multiplication 

 is known as asexual reproduction. In both types of 

 propagation the new individual has its beginning in one 

 or more cells derived from the parent. The living ma- 

 terial is passed from one generation to the next. " All 

 life from pre-existing life " is to-day a rule without a 

 single known exception. 



Spontaneous Generation. — Although it has been 

 known for thousands of years that animals and plants 

 beget their kind, it was also held until recent times that 

 this rule is frequently broken in cases of supposed spon- 



292 



