394 
Gates and Rees . — A Cytological Study of 
of the homologous chromosomes takes place some time later the significance 
of the contraction remains obscure. 
Binucleate pollen mother-cells were found both before, during, and 
after synapsis. It seems probable that such cells arise by the breaking 
down of an incomplete wall between two cells in the archesporium stage. 
Occasionally in diakinesis only eight chromosome bivalents were 
present, and frequently there were only seven or eight separate bodies 
present on the heterotypic spindle. This was found to be due to a temporary 
end to end fusion of certain bivalents, usually the shorter ones but 
occasionally the longest being involved. This phenomenon is also likely 
to disturb Mendelian ratios, causing partial linkage. 
The occasional occurrence of ten chromosomes in diakinesis is due to 
the separation of the members of one bivalent. On the heterotypic spindle 
the chromosomes are greatly condensed and there is usually no indication 
of their bivalent character until the halves begin to separate. Constant 
differences in size can still, however, be observed. 
In the heterotypic telophase the chromosomes are already longitudinally 
split for the homotypic mitosis. 
After the reduction divisions, the cytoplasm of the pollen mother-cells 
begins to constrict at four points, and these constrictions finally meet in 
the centre, cutting the contents of the cell into four parts. The young 
pollen grains so formed alter their shape within the mother-cell wall, 
becoming roughly heptagonal and then secreting a cell-wall. The mother- 
cell wall then breaks down and the wall of the pollen grain ultimately 
becomes remarkably thickened and sculptured. 
The tapetum forms a plasmodium in which the nuclei are more or less 
completely disintegrated, the process of disintegration beginning before the 
break-down of the tapetal cell-walls. The plasmodium flows in among the 
pollen grains, and may contribute directly to the sculpturing of their walls. 
Literature cited. 
Arber, Agnes (1920) : Studies on the Binucleate Phase of the Plant-cell. Journ. Roy. Micro. 
* Soc., 1920, 1-21, 1 pi., 2 figs. 
Beer, R , and Arber, A. (1915; : On the Occurrence of Binucleate and Multinucleate Cells in 
Growing Tissues. Ann. Bot., xxix. 597-8. 
(1919): On the Occurrence of Multinucleate Cells in Vegetative 
Tissues. Proc. Roy. Soc., B, xci. 1—7 r , 1 pi. 
: — (1920): On Multinucleate Cells: an Historical Study (1879-1919). 
Journ. Roy. Micro. Soc., 1920, 23-31. 
Collins, G, N. (1912): The Origin of Maize. Journ. Wash. Acad. Sci., ii. 520-30, 
