102. Evans AND HooKER: PERISTOME IN CERATODON PURPUREUS 
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Fic. 1-10. Cross sections through the opercular region of young capsules, 
showing successive stages of development; peristomial layers stippled, X300. 1 and | 
2, division of segments into quadrants; 3 and 4, establishment of amphithecium 
and endothecium; 5, establishment of inner peristomial layer; 6, division of cells in 
outer amphithecial layer by anticlinal walls; 7, establishment of outer peristomial 
8 and 9, division of cells in inner peristomial layer by anticlinal walls; 10, 
continuation of divisions in outer amphithecial layer and in endothecium. 
thus raising the number of cells in this layer to sixteen (FIG. 6). 
Each of the sixteen cells then divides by a periclinal wall so that 
two layers of sixteen cells each are formed (FIG. 7). The inner 
layer represents the outer peristomial layer and undergoes no 
further divisions. It will be noted that the eight pairs of cells 
in this layer lie opposite the eight cells of the inner peristomial 
layer. In the spore-case the layer homologous with the outer 
peristomial layer gives rise to most of the photosynthetic tissue 
