LIFE HISTORY OF A FERN 1 67 



only one cell in thickness. Because of its flatness this 

 little plant (for such it is) is called a thallus; and because 

 it precedes, in the order of reproduction, the new sporo- 

 phyte, it is called the prothallus (Fig. 126). It is usually 

 possible to divide the prothallus into right and left 

 halves, similar in shape and in other characters, and hence 

 it is said to possess bilateral symmetry. 



