160 Campbell . — On the Development of 
cells. Finally, in each of these four cells, another wall arises, 
so that eight cells in all are formed. While these divisions 
have been going on, numerous radial walls have arisen in both 
tapetum and sporangium-wall, the former being especially 
numerous. 
Shortly after the divisions are completed in the central cell 
and tapetum, the cell-walls of the latter are dissolved. At 
first the group of spore-mother-cells remain together ; but, 
finally, their walls are sufficiently dissolved to completely 
isolate the eight cells, which are then surrounded by the fused 
contents of the tapetal cells. Each of the eight spore-mother- 
cells, as usual, gives rise to four spores. In Azolla these are 
of the tetrahedral type. 
At the time that the spore-mother-cells are about to 
separate, the nucleus is large, but contains but little chromatin 
and consequently does not stain deeply. The nucleolus is much 
less conspicuous than in the earlier stages, and after the cells 
are completely separated it becomes scarcely distinguishable. 
The divisions of the nucleus can be traced without much 
trouble, but owing to the small amount of chromatin, and the 
correspondingly small size of the nuclear filaments, the 
karyokinetic figures are small, and the details difficult to 
follow. As there did not seem to be any deviation from the 
process as seen in the division of the spores of other plants, 
no special observations were made. The nuclear spindle is 
clearly defined, but small (Fig. 18 b) ; after the first nuclear 
division is completed, the daughter-nuclei divide again before 
any division in the protoplasm is to be seen. Then follows 
the simultaneous division of the protoplasm into the four 
young spores (Figs. 19, 20). Of the thirty-two young spores 
thus formed, only one comes to maturity, and the others are 
used up in its growth. 
The spore which is to form the macrospore increases rapidly 
in size. It is at first a thin-walled oval cell, which lies free in 
the enlarged cavity of the sporangium. Examination shows 
that it is surrounded with a thick layer of densely granular 
protoplasm, in which may be plainly seen a number of nuclei, 
