REDUCTION OF PROTHALLUS 381 



among the higher animals, not only in anatomical and 

 histological structure, but in the fact that alternation of 

 generations is universal from Nitella and the mosses up to 

 the highest flowering plants. But as we ascend the series 

 the gamobium sinks from the position of a conspicuous leafy 

 plant to that of a small and insignificant prothallus, becom- 

 ing finally so reduced as to be only recognizable as such by 

 comparison with the lower forms. 



