THE REPRODUCTION OF ANIMALS 213 



Some unicellular organisms appear to reproduce chiefly or exclusively 

 by fission; others reproduce chiefly by conjugation and fission; but many 

 organisms utilize both methods, either alternately or as they are stimu- 

 lated by varying environmental conditions. 



"Potential immortality" of unicellular organisms. Since reproduc- 

 tion is accomplished by division of the entire cell, the parent organism 

 is replaced by two (or more) new "daughter" organisms. Full growth 

 and maturity are followed by division rather than by senility and death. 

 In this sense, a unicellular organism is potentially immortal. Actually, 

 of course, the great majority of unicellular organisms are killed either by 

 other organisms for food or by adverse conditions of their environment. 

 Such death is a matter of chance rather than necessity. 



REPRODUCTION IN THE METAZOA 



The variety and complexity of reproductive methods exhibited by 

 multicellular organisms is much greater than those found in the Protista. 

 Since there are some rather fundamental differences in detail between 

 the reproductive adaptions of the Metazoa and those of the multicellular 

 plants, we have chosen to treat reproduction in the higher animals and 

 plants separately. It would be possible to classify metazoan reproduction 

 into sexual and asexual on the same criterion that we used for the 

 Protozoa, i.e., on whether one or two parents are concerned. Although 

 such a classification would be closer to our usage in everyday speech, a 

 much more fundamental and useful basis for classifying reproduction in 

 the Metazoa is found in the concept of germ and soma. 



Germ and soma. We have seen that one of the chief features of 

 multicellular organisms is the differentiation of their cells and tissues into 

 diverse types, each differentiation accompanied by a corresponding 

 division of labor. In the higher Metazoa there are hundreds of kinds of 

 highly specialized cells, each fitted for some definite and limited function. 

 The most fundamental specialization and division of labor is that be- 

 tween the cells reserved and later specialized for reproduction, and all 

 the remaining cells of the body. The reproductive cells, or germ cells, 

 have no contributing share in the functioning of the body; housed and 

 nourished in it, their sole function is the production of a succeeding 

 generation. All the other cells have some necessary share in the main- 

 tenance and functioning of the body of the individual they compose; they 

 are known collectively as the soma. This distinction between soma and 

 germ becomes progressively more clear-cut and definite as we consider 

 progressively more complex organisms, and it is of great importance in 

 the consideration of reproduction, inheritance, and evolution. 



In the discussion that follows and in the section on genetics we shall 

 use the term asexual reproduction for reproduction from soma tissues 



