CYCADOFILICALES 



39 



contact with the eggs as in a living cycad after the development of 

 its pollen tube. After fertilization, the degree of development of the 

 embryo or the presence or absence of a resting period are not con- 

 cerned in the definition of a seed. 



In the development of the male gametophyte there was probably 

 not much difference between the heterosporous fern and the early 

 seed plant. In the homosporous fern, the development was probably 

 about like that of the homosporous fern of today, with practically 



Figs. 35-36. — Development of an embryo with walls forming from the beginning. 

 Fig. 36, Development of an embryo with a free nuclear stage preceding wall for- 

 mation. 



all of the gametophyte outside the spore. In the early development 

 of heterospory, the gametophyte probably protruded considerably 

 and became green, as it sometimes does in living heterosporous 

 forms. As heterospory advanced, the gametophjrte tissue became 

 more and more included within the spore, protruding only enough to 

 crack the spore coat and shed the sperms (fig, 34) . The pollen tube, 

 at first only a haustorial organ, is a comparatively modern develop- 

 ment. The Cycadofilicales had not reached this stage. 



A guess at the development of the embryo might be hazarded. As 

 the homosporous fern condition was passing into the heterosporous 

 fern condition, while the fertilized egg was still quite small, there 

 may have been no free nuclear period ; but as the megaspore and its 

 eggs became larger, there would be a free nuclear period, followed 

 by the formation of walls and body regions (figs. 35 and 36). 



