CHAPTER 21 



REPRODUCTION 



In the preceding chapters we have con- 

 sidered in some detail the problems of 

 structure and maintenance in the individual 

 animal body. We shall now turn to the 

 problem of reproduction of the individual 

 or maintenance of the race. All animals re- 

 produce, from the simplest protozoan to the 

 most complex mammal, and, furthermore, 

 elaborate provisions are usually made for 

 this all important event. The methods em- 

 ployed by animals today must have had a 

 long and interesting history. 



Undoubtedly, the first forms of life 

 duplicated themselves by some sort of fis- 

 sion, perhaps much like many bacteria and 

 Protozoa do today. This is one form of 

 asexual reproduction, so called because 



there is no sex involved; the cell simply 

 divides into two parts, usually equal in size 

 (Fig. 21-1). When animals became many- 

 celled, some form of asexual reproduction 

 was still retained by many of the lower 

 forms. Hydra, for example, forms buds 

 (Fig. 21-1) which develop into miniature 

 hydras. Planaria, a more complicated ani- 

 mal, still employs fission. With increasing 

 complexity of body structure in higher 

 forms, tliis method of reproduction was lost 

 entirely. 



Sexual reproduction must have been in- 

 troduced very early in the evolution of 

 living things because we find it well estab- 

 lished even among the single-celled plants 

 and animals. There is a series of single- 



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