June, 1923] 
YOUNG — GERM CELLS IN THE POTATO 
329 
and of the chromosomes could not be made out. Efforts to count the 
chromosomes were not very successful on account of their small size and 
crowded condition. It seems probable that the number of chromosomes in 
the somatic cells is either fourteen or sixteen. In the late stages of the 
heterotypic division, it is evident that the chromosomes have been reduced 
in number, and it is no doubt safe to conclude that from that time forward 
the germ cells contain one half the number of chromosomes found in the 
somatic cells. 
After the loss of the nuclear membrane, a typical mitotic spindle is 
formed and the chromosomes come to lie in the equatorial plane of the 
spindle (fig. 3, 4). The chromosomes pass quickly to the poles of the spindle, 
one half going to either pole (fig. 3, 5). Here they assemble in a dense 
chromatic mass (fig. 3, 6) which is soon relaxed and becomes surrounded 
by a nuclear membrane (fig. 3,7). The fibers of the spindle may be seen 
for a time between the daughter nuclei, but disappear before the homoeo- 
typic division. 
Only a short resting period intervenes between the heterotypic and the 
homoeotypic divisions. The nuclear membranes disappear, and the mitotic 
spindles are formed side by side in the cell (fig. 3, 8). These lie at a con¬ 
siderable angle to each other, often crossing at right angles. In this division 
the chromosomes divide, each half passing to a different pole of the spindle 
(fig. 3, p). The daughter nuclei are developed and the spindle fibers 
persist for a time, but no cell wall is formed, all four nuclei lying free in 
the protoplasm of the cell (fig. 3, 10). Soon constrictions appear in the 
surface of the protoplasm (fig. 3, jj), which finally becomes separated into 
four equal parts each containing a nucleus (fig. 3, 12). Owing to the 
tetrahedral arrangement by which one cell is usually more or less concealed 
from view, it frequently happens that only three cells are visible in a tetrad 
group. 
All the pollen mother cells in a locule divide simultaneously. It is usual, 
however, to find the division stages somewhat more advanced at one end 
of the anther than at the other. The division occurs in other anthers in 
the same bud at nearly the same time, so that it is frequently possible to 
find all stages in a series of sections of the same bud. It occasionally 
happens that the homoeotypic division is omitted and that two cells only 
are formed from the mother cell (fig. 3, 13). This occurs particularly in 
the ends of the locules. 
The microspores of a tetrad are at first surrounded by the original walls 
of the pollen mother cells. These walls disintegrate and disappear as soon 
as new walls are formed around the microspores, which now become the 
pollen grains. These cells increase considerably in size and develop thick 
walls (PI. XXVII, fig. 9). The further development of the pollen grains 
was not followed, as the material at hand failed to show the formation of 
generative and tube nuclei. 
