588 Church . — The Structure of the Tha'llus of 
next stage and yet be only 15 mm. in length, owing to the 
compensating contraction of the lower part of the main axis. 
Stage IV. 
A transition now takes place to a more elaborate type of 
cortex. The passage is sudden, and, as a rule, complete in 
all the members of a single whorl, though occasionally mixed 
whorls may be found (Fig. 22). 
Instead of a basal portion of the lowest segment of the 
lateral appendage dilating to form a cortical facet, and bearing 
the rest of the segment and further ramifications as an assimi- 
lating filament, the whole basal segment forms a pedicel 
supporting the segments of the first dichotomy, and it is the 
lower portions of these segments of secondary order which 
now become swollen vesicles and repeat the cortex-formation 
(Figs. 17, 18, 21). The cortex has therefore, as it were, 
moved one degree outwards. The basal segments have again 
a characteristic appearance (Fig. 19), being short with two 
strongly pronounced distal scars, and as before they may 
persist on the main axis long after the loss of the cortical 
dilatations. 
Reversion is general at first. For example, in a well- 
developed young plant (Fig. 17), Stage IV was initiated after 
formation of 42 whorls of Stage III ; but, after 4 whorls 
of the new type had been laid down, a reversion to Stage III 
occurred for 3 nodes ; Stage IV being afterwards resumed 
and its whorls maintained in unbroken succession. 
As the branching of the first dichotomy takes place in 
a horizontal plane, contrary to the method observed by 
Cramer in N. Kelleri , the new cortical whorls will exhibit 
twice the number of facets on the surface of the thallus in 
transverse section. If these are to be of equal size to those 
of Stage III, longer pedicels will be required, and the diameter 
of the thallus will thus be increased. Transitions from one 
stage to the other will therefore result in the production of 
a more or less moniliform appearance (Fig. 17) ; and this 
again is a foreshadowing of what becomes in Cympolia the 
