SPERMATOPHYTES 



197 



the development of cell walls separating the free nuclei; (5) centripetal 

 growth of this tissue until it fills the cavity of the enlarging megaspore 

 (which is now known as the embryo sac, fig. 446). At least two regions 



FIGS. 443, 444- Megasporophylls of Cycas: 443, C. revoluta; 444, C. circinalis. 



may be distinguished in the completed gametophyte; a region of smaller 

 cells at the micropylar end of the embryo sac, in which archegonia are 

 developed; and a deeper region of larger cells, which are nutritive in 



function (compare Sela- 



ginella, p. 136). 



Archegonia. The 

 archegonia vary widely 

 in number, but three to 

 five are most common. 

 The archegonium initial 

 is a superficial cell, which 

 divides periclinally into 

 a primary neck cell (the 

 outer one) and a central 

 cell (the inner one). 



FlG. 445. Mcgusporophylls of Ccratozamia. 



