122 Johnson. — On the Development of the 
account, since it refers to the position of these walls at the 
intersection with the surface, when the thing of importance is, 
as we shall see, their position in the interior. 
The second wall formed in the segment is also a longi- 
tudinal anticline in the secondary marginal cell, and nearly 
parallel to the inner or median border of the segment 
(II, Figs. 4, 6) ; and thus is formed a tertiary marginal cell 
(me. 3 , Fig. 6). This latter is then divided by a transverse 
anticline (/. a. 1 , Figs. 4, 5), the radial wall of Sadebeck, into 
an upper and a lower tertiary marginal cell. Then in each 
of these further section-walls, to the number of three, are 
formed near and parallel to the dorsal and ventral sides 
alternately (III, IV, V, Figs. 4, 5, 7, 8), and there are thus 
formed two marginal cells of the sixth grade in each segment 
(mcl, Fig. 8). In a less frequent type of division only four 
section-walls are formed, and the ultimate marginal cells 
are thus of the fifth grade. 
The Petiole. 
The first nine or ten pairs of segments of the leaf go to 
form the petiole, and the six primary divisions of the segments 
(taking the type where the ultimate marginal cell is of the 
sixth grade) break up into cells, as will now be described. 
About the same time that wall II is formed, there appears in 
section I a pericline (pi. w., Fig. 6), cutting off at the inner end 
a part of the plerome contributed by this section to the 
longitudinal bundle of the petiole. This is followed by a 
longitudinal and radial anticline, the halving anticline, cutting 
the outer cell into two ( h . a., Figs. 5, 7, 8). Each of the other 
sections and the marginal cell in turn cuts off plerome at the 
inner end (pi. w., Figs. 7, 8), but no halving anticline is 
formed in any of them. Then there appears a pericline in 
the outer end of the halves of section I, in the outer ends 
of each of the other sections and of the marginal cell, 
separating a layer of outer cells which give rise to the 
epidermal structures of the petiole from an inner one of cells 
forming the mesophyll. We may for the sake of brevity, 
