Development of the Fern Leaf 15 
segment are simple; those below, by lengthening and sending 
out branches, have begun to develop into the midveins that 
they finally become in the secondary segments, although these 
segments are not as yet formed. In Fig. 1a this development 
of a primary branch into a midvein has but just begun: the 
primary branch may be described either as once forked or as an 
incipient midvein not developed beyond its two first (basal) 
primary branches, which are simple. In Fig. 16 this develop- 
ment has been carried a step farther, in Fig. 2c another step, and 
in other segments of Fig. 2, in some of Fig. 3, and in the second- 
ary segments of Fig. 4, successively still farther. In Figs. 1 
to 3 may be seen the early stages of the similar development of 
the primary branches of the newly-formed midveins into the 
midveins they are to become in the tertiary segments. 
The development of the primary branch of one midvein 
into another midvein with primary branches is brought about, 
as may be seen from the above examples, by the primary branch 
repeatedly lengthening and sending out successive primary 
branches of its own on both sides. This development may 
begin before, during, or after the formation of the incision or 
incisions by means of which the segment that is to contain the 
_ new midvein is formed. At the same time the branches pro- 
duced by the latter midvein may or may not begin to develop like- 
wise. Within reasonable limits, there are apparently no bounds 
to the degree of complexity the primary branches of a midvein 
may attain before or during the formation of the incisions that 
set them apart to constitute midveins. On the other hand, the 
branches produced by a midvein’s primary branch during its 
development into a midvein may, if appearing before the seg- 
