640 Kershaw. — Structure and Development of the 
contained no prothallus, for the development of the embryo-sac had evidently 
been checked. In these cases layers of cells with, the appearance of cork 
have developed in the interior layers of the integument, apparently to cut 
off the aborting region. 
The specimens have a thin integument with, however, a well-developed 
vascular supply, so that the abnormality does not seem to have been caused 
by lack of nourishment as in some cases recorded. 
The cells of the prothallus are of uniform size, and from their arrange- 
ment it seems probable that the tissue has been formed by divisions of 
a peripheral layer of cells which gradually fill up the hollow cavity centri- 
petally. The apex of the prothallus between the archegonia is elevated, 
and although the tissue has shrunk away, a complete impression of its shape 
is seen in the nucellar tissue with which it was in contact. The condition 
of the nucellar cells in this region indicates that there must have been very 
close contact between the two. An upward growth of the prothallus at an 
early stage of development of the archegonia seems constant in Cycads; 
cf. Coulter and Chamberlain. 1 Later, the region of active growth is removed 
to a ring of tissue outside the archegonia, and by rapid cell formation the 
summit of the prothallus becomes sunk in the ‘ archegonial chamber’. The 
archegonium is surrounded by a jacket layer and has a pair of flat neck 
cells (Fig. 20, PI. LXI). The central cell contains relatively little vacuolate 
protoplasm with a central nucleus ; but with as yet no differentiated egg 
membrane. 
The pollen- chamber in these ovules is a large cavity open to the micro- 
pyle above and to the prothallus below (Fig. 21, PI. LXI). Comparison 
with pollinated ovules suggests that in a normal ovule of this age the passage 
would not be completely formed. It seems advisable to call the whole 
cavity the pollen-chamber and to distinguish it into the upper pollen-chamber 
and lower pollen-chamber ; such a division seems functionally a- natural one. 
The tipper pollen-chamber is the narrower part of the cavity in the pointed 
tip of the nucellus, which extends for some distance up the micropylar tube. 
Its walls are thin and hardened and taper off to a fine point. The lower 
pollen- chamber is the large cavity in the wider part of the nucellus beneath 
the tip. 
The older abnormal ovules are about 8 mm. long and therefore twice 
the size of those described above. Two of them are represented in outline 
in Text-figs. 15 and 1 6. The integument and nucellus are normally developed 
and the two parts of the pollen-chamber are seen. The walls of the upper 
cavity are hard and cutinized and give it the appearance of a dark, pointed 
cap, similar to that seen in the old pollinated ovule. The lower pollen- 
chamber is abnormal in its relation to the prothallus, which is of exceptional 
appearance, often being completely twisted out of shape. The form and 
1 Coulter and Chamberlain : Morphology of Gymnosperms. Univ. of Chicago Press, 1910. 
