996  KirKwoop: POLLEN-FORMATION IN CUCURBITACEAE 
the time of the division. ‘At about the time of synapsis the cyto- 
plasm undergoes an interesting change. At this time the position 
of the nucleus in the cell is more or less eccentric and the nucleolus 
and the chromatin mass are usually on the side of the nucleus 
nearest the cell-wall. The cytoplasm, which up to this time has 
shown no special differentiation, now displays a number of fine 
fibers running tangent to the nucleus along the arc lying opposite 
the chromatin mass. These fibers may be traced distinctly to the 
periphery of the cytoplasm, where they appear to connect with the 
plasmatic membrane. The cell at this time presents an appearance 
much as if the nucleus by rapid expansion had placed under tension 
some of the fibers of the cytoplasmic reticulum (FIGURE 24). 
Toward the periphery of the cell an apparent branching of the fibers 
was often noticed and frequently thicker portions which stained 
darkly were plainly seen. If there is any significance in these fibers 
it is not apparent. As the nucleus resumes its position at the 
center of the cell at the close of the synapsis period they are no 
longer visible but the meshes of the cytoplasmic reticulum appear 
drawn out in a radial direction from the nucleus, which is a char- 
acteristic condition just preceding the prophase. There is, how- 
ever, no indication of fibers such as appear in Codaea," Larix,’ or 
Equisetum.™ As these changes take place there is a drawing 
away of the ectoplasm or “ Hautschicht’”’ from the cell-wall, so 
that the mother-cells, instead of remaining angular, become prac- 
tically spherical. Cannon* noted the same phenomenon in the 
spore-mother-cells of cotton and regarded it as the normal and 
regular process in those cells. The same conclusion seems to be 
justified here by the evidence at hand and by the subsequent his- 
tory of the cells arising from the ensuing divisions. The spherical 
form is probably an advantage in the exact division of the cells 
into tetrads after the manner in which they usually occur. The 
nucleus, which in the earlier stages shows a perfectly even curva- 
ture of its membrane, now begins to contract and its outlines are 
less regular. This irregularity becomes more and more pro- 
nounced until the membrane is finally dissolved in the prophase 
of the first division. 
In FIGURE 23 are represented certain cytoplasmic bodies whose 
history we have made an effort to follow. At a certain stage in 
