CHAPTER 9 



REPRODUCTION 



The functions of metabolism and irritability have been exam- 

 ined in our study of a vertebrate animal and of unicellular organ- 

 isms. There remains the third great bodily function, reproduc- 

 tion. We may now consider this in a similar manner, by examin- 

 ing the reproductive processes of protozoa and by extending the 

 examination to multicellular animals. The protozoa arise from pre- 

 existing organisms like themselves, as do higher types of animal 

 life. Both sexual and asexual reproduction occur in protozoa, 

 asexual reproduction being effected by cell division, and sexual 

 reproduction by the union of gametes which may or may not be 

 sexually differentiated as microgametes and macrogametes. In 

 the metazoa, sexual reproduction also occurs by means of gametes, 

 the ova and spennatozoa, while asexual reproduction, by processes 

 of budding and fission, occurs in some many-celled annuals. In 

 addition, there is in metazoa the process called development, 

 which may be distinguished from reproduction for purposes of 

 definition, although the two are inextricably related. The union 

 of ovum and spermatozoon is obviously comparable with the 

 union of conjugating cells in protozoa to form a new individual; 

 but nothing in the protozoan cycle is exactly comparable with the 

 development of a many-celled body from the zygote by cell division 

 and differentiation, except the division of the protozoan cell and 

 the colony formation that appears in a few species. Hence devel- 

 opment occurs only in metazoa, while reproduction occurs in 

 both protozoa and metazoa. By making this tlistinction, the 

 process of reproduction may be considered in the present chapter 

 a,nd the study of development deferred until we have completed 

 our examination of representative many-celled animals. 



The Reproductive Cycle 



The Cell Cycle in Single-celled and in Many-celled Organisms. 

 ^The resemblances between the reproductive processes through- 

 out the Animal Kingdom may be shown by a schematic represen- 



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