WILLISTON: DISCOID GEMMAE IN RADULA 333 
median wall before beginning to function as apical cells (FIG. 15), 
and those in which the establishment of the two apical cells is not 
simultaneous (FIG. 16). In any case the shape and size of the 
two sides of the gemma are not noticeably affected (FIG. 16 and 17). 
The two lower quadrants of the gemma mother cell do not produce 
apical cells but divide by longitudinal walls followed by transverse 
walls. After the apical cells in the upper half of the gemma 
cease to function, the whole gemma increases in size by the same 
method of cell division, followed by the inner or lower half from 
18 : : p 
pe 28 
Ue & 
24 
of oe 15-26, Radula flaccida, Gemmae with irregularities in the formation 
medi apical cells. 15, gemma which has cut off a segment on each side toward a 
an line before the apical cell has begun to function, X400. 16, gemma in which 
median line, and ceased to function. The apical cell on the right was 
m the outer half of the original triangular quadrant and is still functioning 
ma in which the apical cell on the left has ceased to function, while that on th 
ig Cutting off segments, 533. 18 and 19 show the method of the develop- 
tharet 
17, gem 
Tight ig 
develop m the sequence of cell division, X 400. 24, dorsal view of later stage in 
ieW, devant the leafy shoot, x533. 25 and 26 show the leafy shoot, ventral 
in dextrorse and sinistrorse spirals, X 800. 
. 
