KIRKWoOD: POLLEN-FORMATION IN CUCURBITACEAE 233 
fibers from the first division frequently persist through the second 
and after the grand-daughter nuclei are formed they may be seen 
connected by fibers of both preceding mitoses. The spindles of 
the second mitosis lie in parallel planes usually at right angles to 
each other but often parallel. Thus the tetrads usually appear in the 
ordinary pyramidal arrangement but sometimes they are all 
in one plane. The division of the cytoplasm takes place by con- 
striction, a narrow groove marking off the line of division. This 
groove deepens until the spores are fully separated. 
The nuclear contents of the cells under consideration have 
been difficult objects for study on account of the minute size of 
the chromosomes. The nucleoli are often large and conspicuous 
and at certain stages are apparently vacuolated. In the period of 
development just succeeding the cutting off of the tapetal cell, the 
primary sporogenous cell is distinguished from its neighbors by its 
dense and darkly-staining cytoplasm. The nuclei of the primary 
tapetal and primary sporogenous cells are at first to all appearance 
similar. In both the chromatin is arranged about the nuclear 
wall in irregular masses so that the nuclear vacuole at first appears 
to contain only a large nucleolus. In this case, in which the 
spore-mother-cell is derived directed from the primary sporogenous 
cell, the period of growth is begun at once and the nuclear changes 
keep pace with the development of cytoplasmic structures. The 
linin of the nucleus soon becomes more apparent and its fibers 
may be seen passing in different directions through the nuclear 
vacuole. The chromatin soon shows a tendency to cling less 
closely to the nuclear wall and it may be seen strung along the 
linin network in small irregular masses. A marked increase in 
the quantity of chromatin is characteristic of this stage and the 
lines of the nuclear reticulum are thickly beaded with chromatin 
granules. 
The reticulum now resolves itself into a continuous thread 
Which gradually becomes more uniform. While this change 1s 
taking place all the chromatin with the nucleolus becomes crowded 
into one side of the nuclear vacuole and synapsis results. Often 
the chromatin in synapsis may be seen as a much coiled thread 
with many sharp convolutions, sometimes extending almost across 
the nucleus, During this process the nucleolus remains undi- 
minished in size and staining properties. 
