MORPHOLOGY 
The primary neck cell divides vertically, and these two neck cells, 
lying side by side, are constant among the Cycadales. The central 
cell then begins a remarkable enlargement, and becomes invested by 
a special jacket of cells, known as the archegonial jacket, which func- 
tions as a nutritive layer. Finally the 
nucleus of the central cell divides into the 
ventral nucleus, which soon disorganizes, 
and the egg nucleus, about which the cyto- 
FlG. 446. Diagrammatic 
section of ovule of Dioon, show- 
ing the thick integument free 
from nucellus at beak (in which 
the pollen chamber is evident, 
but not open); the three layers 
of the testa, outer fleshy (trav- 
ersed by vascular strands), stony 
(thick and shaded), and inner 
fleshy (distinct in region of beak, 
but merged with nucellus be- 
low, and traversed by vascular 
strands); the embryo sac con- 
taining the female gametophyte 
(endosperm), in which two 
archegonia are present. After 
CHAMBERLAIN. 
FIG. 447. Micropylar end of a mature 
archegonium of Dioon, showing the two neck 
cells, the ventral nucleus (in the apex), and the 
egg nucleus (below). After CHAMBERLAIN. 
plasm is organized to form the egg (fig. 447). The notable feature of 
this archegonium, in contrast with the archegonia of bryophytes and 
pteridophytes, is the complete elimination of neck canal cells. The 
cycad egg and its nucleus are the largest known among plants. As the 
