94 
Barr ait , — The Origin of the 
continue segmenting - , but it is generally possible, for quite a long time, 
to trace the outlines of these groups which originate from separate initials. 
The cells of the periblem do not divide so early, but enlarge to keep pace 
with the increasing bulk of plerome, and later divide by anticlinal walls, 
thus preserving the original number of layers. 
The air-channels begin to form in the periblem as intercellular spaces at 
the angles between cells of different layers. As a result, one finds rings 
of small air-spaces appearing between successive layers of periblem cells. 
Fig. 2. Transverse section’of same tip at region where the longitudinal section ends. 
Such spaces are never found between the dermatogen and the first layer 
of the periblem ; moreover, the ring of canals between the first and second 
periblem layers develops a little later than the ones within. As can be seen 
by reference to Fig. 3, there are four rings of canals developing. 
At this stage the cells of the periblem begin to divide, and the subse- 
quent arrangement of the cortical cells and air-canals is dependent on 
the direction in which these periblem cells segment. The innermost layer 
becomes divided by periclinal walls which are laid down in such a way as to 
