^9 



Figs. 27-31. — Hypothetical development of a heterosporous carboniferous fern an- 

 cestor of the Cycadofilicales. Fig. 27, a megasporangium with spore mother-cells, at 

 this stage indistinguishable from a homosporous sporangium. Below are aborting spo- 

 rangia and the sorus is surrounded by an envelope which might be called an integument. 

 Fig. 28, one mother-cell has divided. Fig. 29, one megaspore has germinated and has 

 produced two eggs, and has developed a heavy spore coat. Fig. 30, the female gameto- 

 phyte is protruding and has an embryo. The sporangium wall has been ruptured. Fig. 

 31, the megaspore, with its embryo, has been shed from the sporangium and is germinat- 

 ing outside the sporangium. Therefore it has not quite reached the seed stage. 



