Allen . — Nuclear Division in the 
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pollen mother-cells may be found by the end of the first week of June; 
and stages in their division during the rest of June and the first week 
of July. The anthers were removed from the bud, usually cut transversely 
in half, and immersed at once in the fixing fluid. Collections were made 
during each of five summers, from 1899 to 1903 inclusive. 
Sections were cut from three to thirty micra in thickness, and before 
staining were immersed from fifteen to thirty minutes in a solution of hydrogen 
peroxide. For the study of the cytoplasmic structures and of the processes 
connected with spindle-formation, sections of 5-6 /x thick, stained with 
Flemming’s triple stain, were found of most use. For the study of the 
chromatin, the best results were obtained from sections from twelve to 
thirty micra in thickness, stained with Heidenhain’s iron-alum-haema- 
toxylin. The two staining methods, however, supplement each other, and 
a comparison of sections from the same anther, some treated in one way, 
some in the other, is always helpful. 
DESCRIPTION OF OBSERVATIONS. 
The Heterotypic Division. 
The Period of the Nuclear Reticulum. 
The divisions in the sporogenous tissue of the anther sac which precede 
the formation of the pollen mother-cells go on very rapidly, accompanied 
by growth of the cells and by a corresponding increase in size in all 
dimensions of the anther itself. A section taken at any stage during this 
series of divisions shows cells and nuclei in all stages of division. Even 
when the majority of the cells in the sac have entered upon the long period 
of growth which characterizes the pollen mother-cells, occasional figures 
may be found showing stages in the last preceding division. But soon 
these isolated divisions are completed, and the sporogenous tissue then con- 
sists of a cylinder, from six to eight cells in diameter, all the cells being in 
substantially the same stage of development. A section taken at any time 
from this period down to the appearance of the multipolar spindle-figures 
shows a striking uniformity as to stage of development among all 
the mother-cells of a sac. Apparently the processes concerned in the 
development of the cell and in the preparation for the heterotypic division 
go on very slowly and at about the same rate in all the cells of a sac. 
In some cases a certain amount of progress may be noted in passing from 
one end of a sac to the other ; more often there is a sort of rhythmical 
variation, so that at two or three regions of the sac the greatest progress 
has been made, while in moving from one to another of these points we pass 
through a region showing gradually earlier, and then gradually later stages, 
