384 
CLIFFORD H. FARR 
firms the general opinion that the plane of cell division is determined by 
the direction of the spindle (Giesenhagen, 7), or, perhaps, that they 
are consequences of the same factors. 
Regarding the formation of the furrow, the observations on living 
cells confirm the statement of Guignard (8) that an incipient furrow 
is developed during the first division, which is, however, arrested and 
only completed after the homoeotypic karyokinesis has taken place. 
In some cases the heterotypic furrow seems to be preceded by an equa- 
torial differentiation, but this always disappears before cytokinesis is 
effected. During the homoeotypic nuclear division the heterotypic fur- 
row remains arrested, but after the four nuclei are organized this furrow 
completes the division of the cell. At the same time the homoeotypic 
furrows are developed across the equators of the homoeotypic spindles 
at right angles to the heterotypic furrow, resulting in the mother cell's 
becoming subdivided into four microspores (fig. 10). Whether the 
heterotypic furrow completes the division of the mother cell into two 
parts before the homoeotypic furrow is complete is not easy to deter- 
mine from the living cells, and consequently this question will be left 
until the study of the prepared slides is discussed. 
There are certain terms that are necessary in referring to various 
parts of the mother cell during the reduction divisions; and it is de- 
sirable that the meaning of these words be clearly understood. The 
furrow which begins during the heterotypic division and is completed 
after the second mitosis is called the heterotypic furrow, even though 
the later stages of its development follow the homoeotypic karyokinesis. 
The spindle between the two daughter nuclei of the first division is 
called the primary heterotypic spindle; and the area which it crosses is 
the heterotypic equator. The spindle which crosses this equator after 
the second nuclear division is known as the secondary heterotypic 
spindle. The spindles connecting the sister nuclei of the second 
division are the homoeotypic spindles, and their equators are crossed by 
the homoeotypic furrows. The region of cytoplasm on the opposite 
side of each nucleus from the central spindle is the polar region, and 
that beside the nuclei and the spindles is the lateral region. 
Nuclear and Cytoplasmic Phenomena 
The stamen enlarges prior to the initiation of the reduction divisions, 
so that the pollen mother cells are free within the pollen sacs before 
