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). 
FIG. 445. Megasporophylls of Ctratuzamia. 
