ASEXUAL 



SEXUAL 



CblamydomoDOS 



Heferogamy 



Porameeium 



Fig. 21-1. Asexual reproduction occurs among the Protozoa and lower invertebrates. Sexual 

 reproduction was initiated among some of the simpler Protozoa where it progressed from 

 simple fusion of two similar cells to the union of highly diverse reproductive cells, the 

 sperm and egg. The steps which brought this about may have been similar to those outlined 

 here, using Protozoa that are living today. 



