1912] STEVENS—HETEROSTYLOUS PLANTS 289 
THE REDUCTION DIVISION OF THE POLLEN MOTHER CELLS 
Each loculus contains usually a single row of 8 or to pollen 
mother cells. All the cells of each loculus apparently pass through 
the different stages simultaneously, and there is no evidence of a 
regular basipetal succession in their development, such as has been 
reported in numerous cases. It was necessary, therefore, in order 
to determine the succession of the various stages, to compare care- 
fully the cells of different loculi, using their size and the condition 
of their cytoplasm as a check upon the order of the phases shown 
by the nuclei. Considerable variation in different loculi of the 
same flower is common. | 
PROPHASE.—The pollen mother cells first become distinguish- 
able by their increased size and the possession of a large nucleus, 
containing a single large, dark staining nucleolus (figs. 1, 2). This 
nucleolus is surrounded by a clear zone which is apparently not 
affected by any of the stains used. In a few cases two such nucleoli 
were observed in one nucleus, each surrounded by the colorless 
area just described. The nature of this clear space seems to be 
rather uncertain. It has been figured frequently, and is regarded 
by some writers as a constant structure. Martins MANo (24, 
p. 60) speaks of it as the ‘“‘peri-nucleolar vacuole.”” STRASBURGER, 
however, considers that the appearance is due to reagents (37, 
P- 519). 
Surrounding this clear zone is the nuclear reticulum, consisting 
of very delicate indefinite threads which do not take the chromatin 
stain. This “linin” network contains scattered dark staining 
bodies which are apparently rather irregular in number and do not 
seem to occur in pairs. They thus furnish no support for the 
attractive prochromosome theory of ROSENBERG (31, p. 25) and 
others. 
SyNapsis.—The recent work of LAwson (22) has again raised 
the question as to whether the phase of the nucleus preceding the 
reduction division, characterized by an apparent condensation and 
contraction of the chromatin on one side of the nuclear cavity, is 
a real contraction. For some time this condition was regarded as 
an artifact, but it has been observed in living material by several 
investigators, and its occurrence, at least in some forms, is now 
