66 Campbell. — On the Prothallium and Embryo of 
surface of the cushion back of the apex, but form a row on 
either side of the midrib, while from the central part root- 
hairs alone are produced. O. cinnamomea corresponds almost 
exactly in this respect to O. regalis ; but in O. claytoniana 
the archegonia are formed in greater numbers and extend 
further toward the centre of the midrib, so that in their distri- 
bution, as well as in the broad forward part of the midrib, 
this species resembles the Polypodiaceae more than do the 
others. 
In studying the development of the archegonium, the 
prothallia were imbedded in paraffin and sections made at right 
angles to the midrib. In this way many of the archegonia are 
cut vertically, and no difficulty was experienced in finding all 
stages. 
No correspondence could be detected in their origin with 
the early divisions of the segments of the apical cells, as is 
the case in some Polypodiaceae, and the youngest are formed 
some distance back of the apex. While formed in ap- 
proximately acropetal succession, new ones arise also among 
the older ones. 
The mother-cell of the archegonium (Fig. 61 m) is scarcely 
distinguishable either in form or contents from its neighbours, 
and it is not always easy to decide whether a special cell will 
develop into an archegonium or not. Sometimes it is deeper 
than the secondary cells, and the nucleus somewhat larger ; 
but its small size and indifferent character offer a strong con- 
trast to such specialized forms as Isoetes for instance, where 
from the first the archegonium-mother-cells are immediately 
recognizable. 
The first division wall is at right angles to the axis of the 
archegonium, and divides the mother-cell into an inner and 
an outer cell (Fig. 68). The next division is usually parallel 
with the first, and divides the inner cell into two, so that as in 
other ferns the young archegonium consists of three super- 
posed cells (Fig. 62). In some cases, however, the division of 
the inner cell does not take place, or it may occur simulta- 
neously with the first division in the outer cell. Fig. 68 shows 
