THE EVOLUTION OF STRUCTURES 155 



the female gametophyte. We venture to hazard a guess 

 at missing links in the phylogeny and believe that the 

 guess is not far from what would actually be found, if 

 fossils of early heterosporous pteridophytes and the 

 earliest gymnosperms could be secured. That the 

 heterosporous condition, with its large female spores 

 and smaller male spores, has been derived from the 

 homosporous condition, in which the spores are all alike 

 and small with no differentiation of sex, is too obvious 

 for argument. When such spores fall upon a moist sub- 

 stratum they germinate, developing a gametophyte 

 which immediately protrudes from the spore and takes 

 on a green, flattened form producing eggs and sperms. 

 But when the spores are differentiated into larger female 

 and smaller male spores there is little or no protrusion 

 at germination, the gametophyte developing within the 

 spore and having necessarily a more or less spherical 

 form. Being protected from light by the thick spore 

 coat, there is little or no development of chlorophyll, so 

 that the gametophyte is nearly colorless. The spore is 

 large in comparison with the nucleus and is densely 

 packed with food material. The large size of the spore 

 would make it difficult for the nucleus to segment the 

 large mass into two cells, and the food material would 

 shorten the interval between nuclear divisions. As a 

 result of the large mass and the rapid sequence of divi- 

 sions, walls might very naturally fail to be formed, and 

 nuclear divisions without accompanying cell walls — free 

 nuclear divisions — would continue until the mass about 

 each nucleus became small enough to be segmented; 

 then walls would begin to appear, and the gametophyte 

 would become cellular. We beheve that this has been 



