LIFE HISTORY OF A FERN 167 
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. 
