4 
EVANS AND HOOKER: PERISTOME IN CERATODON PURPUREUS 103 
including the large intercellular space. The condition shown in 
FIG. 7 corresponds closely with figures by Kienitz-Gerloff (’78, 
pl. 1, f. 256) and Kuntzen (’12, f. 10), representing sections through 
the spore-case. The eight cells of the inner peristomial layer 
now proceed to divide in a very peculiar way. In each of these 
cells an anticlinal wall appears (FIG. 8), cutting off either to the 
right or left a small cell from one fourth to one third as wide 
as the parent cell. This is followed by a second anticlinal wall 
cutting off a similar small cell on the other side of the parent 
cell (FIG. 9). In this way the inner layer becomes divided into 
- twenty-four cells, each group of three corresponding with one of 
the pairs of cells in the outer layer. No further divisions are 
undergone by these twenty-four cells. The arrangement of cells 
just described seems to be even more constant than Kienitz- 
Gerloff implies, since he states that the number of cells in the 
inner layer may vary from twenty to twenty-four. 
The endothecium and the peripheral layer of the amphithecium 
play no direct part in the development of the peristome, and yet 
an account of the cell divisions that take place in them will not be 
wholly out of place. The divisions in the peripheral layer are 
not altogether definite. After the sixteen-celled stage, still shown 
in FIG. 8, the cells divide by anticlinal and periclinal walls without 
following a rigid system, adjacent cells often dividing in different 
ways (FIG. 10). In all cases, however, the final condition is° 
essentially the same, three concentric layers of cells being pro- 
duced. The outermost layer is composed of (approximately) 128 
cells, the middle layer of 64 cells, and the innermost of 32 cells. 
In the region of the annulus, however, only two layers of cells are 
formed, thus allowing for the extraordinary development of the 
annular cells (FIG. 13, 25, 26). The peripheral layers of cells 
form the operculum, the external walls of the outermost layer 
becoming strongly thickened. 
The divisions in the endothecium of the opercular region are 
essentially like those in the spore-case, as described and figured 
by Kuntzen (’12). After the four-celled stage, shown in FIG. 8, 
division takes place according to the “fundamental square” 
method, eight outer cells and four inner cells being thus formed 
(FIG. 9). The inner cells repeat this method of division (FIG. 10), 
