Sperm-Cells in Asclepias Cornu ti , Decoisne. 139 
together. Faint traces of the delicate wall that originally 
surrounded the smaller cell remain, even after the sperm-cells 
have advanced some way down the tube (Fig. 36), but finally 
the membrane completely disappears. Eventually the sperm- 
cells pass the tube-nucleus (Fig. 37 ), and are carried through 
the micropyle to the cavity of the ovule by the pollen-tube, 
which penetrates as far as the synergids. The fate of the 
tube-nucleus was not determined. 
Conclusion. 
There now remains the question first raised by Wille as to 
the true nature of the structures referred to above as pollinium- 
cells, and the homologies of the primary pollinium-cells. 
Juel’s work on Carex has shown us that it is possible to have 
in the higher plants a structure physiologically a pollen-grain 
but morphologically something quite different. In fact, three 
alternatives may be stated as possible in the development of 
the sperm-cells in Angiosperms. 
1. The normal development as described for most genera 
where the pollen-mother-cell, by two divisions at right angles 
to each other, gives rise to four pollen-grains, situated at the 
corners of a tetrahedron. Each grain ultimately develops 
two sperm-cells and a vegetative cell. 
2. Division of the nucleus of the pollen-mother-cell into four 
nuclei, three of which abort, as in Carex. In this case only one 
pollen-grain develops from each mother-cell. 
3. The development of the sperm-cells from the pollen- 
mother-cell without the so-called tetrad division of the latter, 
as suggested by Wille and others for Asclepias . This would 
be analogous to the development of the embryo-sac directly 
from the mother-cell in Lilium , but has not yet, so far as the 
writer knows, been demonstrated for any plant. 
The second case is at once ruled out for Asclepias, since 
abortion of cells was not found at any stage in the develop- 
ment of the sperm-cells. Which of the two remaining cases, 
then, obtains here? Are the pollinium-cells pollen-mother- 
cells, as Wille and others have held, or true pollen-grains, as 
