June, 1923] 
YOUNG-GERM CELLS IN THE POTATO 
333 
collapses and passes into a state of disintegration. When the pollen mother 
cells are involved (PI. XXVII, fig. 4), the cytoplasm shrinks away from the 
cell wall and appears as an elongated, shriveled mass within the cell, which 
takes a deep stain. The tapetal cells also collapse and the tissue dis¬ 
integrates. 
The period of tetrad-formation seems to be a critical time in the de¬ 
velopment of the pollen. Many instances were found in which degenerative 
changes had taken place at that stage. If the blasting occurs at the time 
of, or immediately following, the heterotypic division, the changes resemble 
somewhat those which take place in the pollen mother cells (PI. XXVII, 
fig. 5 )- The cell contents become shrunken and stain deeply, and the 
daughter cells may become separated before disintegration takes place. If 
disintegration is delayed until after tetrads have been formed, a quite 
different series of changes is noted (PI. XXVII, fig. 6). The tetrads remain 
in the original walls of the mother cells, and little or no shrinkage is evident. 
The cell contents stain lightly, numerous vacuoles appear, the cells at last 
lose their identity, and disintegration ensues. 
It sometimes happens that no change is evident until after the walls 
of the pollen mother cells have disappeared and the cells of the tetrads 
have become rounded as pollen grains. Degeneration at this stage is 
marked by a cessation of growth and by the failure of the pollen grains to 
develop the characteristic thick coat (PI. XXVII, fig. 7). The cell contents 
stain lightly and contain numerous vacuoles. Afterwards the entire cell 
breaks down and disintegrates. In case the thick coat has already formed 
around the pollen grain (PI. XXVII, fig. 8), the latter does not reach the full 
size of the normal grain and is found at blossoming time either empty or 
containing a small amount of material in an advanced stage of disintegration. 
Degenerative changes in the ovules generally accompany those in the 
anther, particularly where such changes take place in the early stages of 
pollen development. Change in the archesporial cells is characterized by 
cessation of growth and by a light staining reaction of the cell contents. The 
archesporial cells, however, do not disintegrate previous to the general 
breaking down of the ovules and placenta, which breaking down does not 
take place until the buds have dropped. 
In the late megaspore stage or that of the one-nucleate embryo sac, when 
the integument has nearly covered the nucellus, blasting is accompanied by 
the shriveling of both the megaspore or embryo sac and the cells of the 
nucellus which surround it (PI. XXVII, fig. 1). The changes which take 
place at later stages are quite similar, and there is nothing like the variety of 
changes which have been noted in the contents of the anther. The embryo 
sac and the remains of the surrounding nucellar tissue shrink away from the 
walls of the integument, stain deeply, and present a shriveled, disorganized 
appearance (PI. XXVI, fig. 7, and PL XXVII, fig. 2). The shrinkage begins 
at the micropylar end of the embryo sac, the tissues remaining attached at 
