44 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



of the parent's body in two. Previous to this splitting in two 

 there may be a temporary fusion for the purpose of a mutual 

 exchange of part of the body substance, or a permanent con- 

 jugation of two individuals. The process of reproduction 

 among the many-celled animals is far more complex, and certain 

 particular organs of the body of complex structure are specially 

 devoted to this function. The results of the process, however, 

 are the same as among the lower animals, namely, the produc- 

 tion of new individuals. The manner in which the reproduc- 

 tive process is carried on, and the number of new individuals 

 produced by a single parent individual, may and do vary much 

 among different animals. 



Among some of the -simpler many-celled animals the new 

 individual is sometimes produced by the growth of an external 

 bud, or by the splitting off of a small part of the parent's body, 

 a process much like the fission, or splitting in two, of the one- 

 celled animals. But this is an unusual method, and possible 

 to comparatively few animals. In almost all cases the young 

 come from eggs, or ova, which are produced inside the body of 

 the mother. These eggs usually issue from the body before 

 hatching, but in some animals, as all the Mammalia, the young 

 develop from the ova inside the body and are born as active 

 free animals, resembling the parent more or less in appearance 

 and structural character, although of course much smaller. 



In all cases the young animal has to undergo a certain amount 

 of development and growth, which extends over a longer or 

 shorter period of time, before it is really like its parent, that is, 

 before it is a fully developed, full-grown individual. No 

 animal is born fully developed; it is born from the body of its 

 mother or hatched from its egg in an immature condition, and 

 growth and change are necessary before we have a fully devel- 

 oped rabbit or robin, or any other kind of animal. 



But when we begin the study of the life history of the new 

 animal with the time of its emergence from the body of the 

 mother or from the egg, we are not beginning at the beginning. 

 When we first see the new animal it is already of appreciable 

 size and complex structure. But at its very beginning inside 

 the body of the mother it is, in every case, simply a single cell. 



