CHAPTER XV 



THE REPRODUCTION OF ANIMALS 



Although there are many essential similarities between the reproductive 

 processes of the Protozoa and the Metazoa, the differences between a 

 unicellular and a multicellular scheme of organization are nowhere more 

 marked than in the different problems that they entail for the reproduc- 

 tion of new individuals. 1 As a consequence, it will be simpler and more 

 accurate to consider the reproductive processes of the two groups sepa- 



Fig. 15.1. Binary fission in Amoeba. (Redrawn after Woodruff.) 



rately. Even if, following common usage, we employ the terms sexual 

 and asexual in both Protozoa and Metazoa, it will be necessary to define 

 them quite differently in the two groups. 



REPRODUCTION IN THE PROTOZOA 



Asexual reproduction. Perhaps the simplest of all types of reproduc- 

 tion is the fission exhibited by many of the unicellular organisms. When 

 an amoeba reaches its maximum size, it divides into two "daughter" 

 amoebae of approximately equal size, and these, when grown to full 

 size, will each, in turn, divide into daughter cells. In Amoeba and most 

 protozoa, this division appears to be essentially mitotic, the outwardly 

 visible cytoplasmic division preceded by a more or less precise nuclear 



1 The fundamental difference involved is discussed under Germ and soma on p. 213. 



211 



