

SECTION I. 



THE REPRODUCTIVE PROCESSES. 



ALL of the functions of the animal body economy which have as yet 

 been considered have dealt solely with the preservation of the individual. 

 Through the exercise of the reproductive function is accomplished the 

 preservation of the species. The duration of the life of any single 

 animal is a limited one, and if, therefore, each species did not possess 

 the power of reproducing itself in a new and similar individual the 

 species would eventually die out. 



The mechanism by which reproduction is accomplished greatty 

 differs in different groups of the animal kingdom. In all vertebrates it 

 is accomplished by the union of two individuals of opposite sexes, male 

 and female. In a large number of the invertebrate animals the two 

 sexes, or at least the two sexual organs, are found united in the same 

 individual, and the different acts of generation are thus accomplished in 

 the body of the same animal; while the mode of reproduction offers 

 a strong analogy to that of the vegetables, which, likewise, contain 

 within the same floral envelope the organs of the two sexes. In other 

 animals still more imperfect a mode of generation may be observed 

 which is analogous to that of cryptogamous plants. Here no organs of 

 generation can be detected, and reproduction is accomplished by sep- 

 aration of parts of the parent body which possess the power of 

 development and growth. Sometimes the germ detaches itself from 

 the individual in the form of a vesicle, which passes through all the 

 phases of development. Such a mode of reproduction is spoken of as 

 generation by spores. At other times a bud may be noticed to form 

 from within or without the animal, which, after having acquired a more 

 or less complete development, separates itself from the parent, and after 

 the separation continues to grow and develop into a new animal. Such 

 a mode of reproduction is spoken of as gemmiparous generation. 

 Finally, the new organism may develop from the parent organism by a 

 simple process of detachment of a part of the parent, after the separa- 

 tion the detached portion forming a new animal, while the parent 

 replaces the part which was lost. Such a mode of reproduction is 

 spoken of as generation by fission. 



In all animals provided with organs of generation, whether borne 

 by different individuals or united in the same individual, generation is 



(903) 



