elaginella, through the connecting link of the conifers, where the
ovule is of similar origin and position to the macrosporangium of the
Lycopodiaceae. But the formation of the macrospore or embryo-sac is
simpler than the corresponding process in cryptogams. It arises by a
simple enlargement of one cell of the nucleus instead of by the division
of one cell into four, each thus becoming a macrospore. At the top of
this macrospore or embryo-sac two or three germinal vesicles are formed
by free cell formation, and also two or three cells called antipodal
cells, since they travel to the other end of the embryo-sac; these
latter represent a rudimentary prothallium. This formation of germinal
vesicles and prothallium seems very different from the formation of
archegonia and prothallium in Selaginella, for instance; but the link
which connects the two is in the gymnosperms, where distinct archegonia
in a prothallium are formed.

Thus we see that the flowering plant is essentially the equivalent of
the asexual fern, and of the sporogonium of the moss, and the pollen
cell and the embryo-sac represent the two spores of the higher
cryptogams, and the pollen tube and the germinal vesicles and antipodal
cells are all that remain of the sexual generation, seen in the moss as
a leafy plant, and in the fern as a prothallium. Indeed, when a plant
has monoecious or dioecious flowers, the distinction between the asexual
and the sexual generation has practically been lost, and the
spore-bearing generation has become identified with the sexual
generation.

Having now described the formation of the pollen and the germinal
vesicles, it only remains to show how they form the embryo. The pollen
cell forms two or three divisions, which are either permanent or soon
absorbed; this, as before stated, is the rudimentary male prothallium.
Then when it lies on the stigma it develops a long tube, which passes
down the style and through the micropyle of the ovule to the germinal
vesicles, one of which is fertilized by what is probably an osmotic
transference of nuclear matter. The germinal vesicle now secretes a
wall, divides into two parts, and while the rest of the embyro-sac fills
with endosperm cells, it produces by cell division from the upper half a
short row of cells termed a suspensor, and from the lower half a mass of
cells constituting the embryo. Thus while in the moss the asexual
generation or sporogonium is nourished by the sexual generati