Those which are of two kinds are, first, a generally aggressive and motile fertilizing or so-called "male cell," called in its typical form an antherozoid; and, second, a passive and motionless receptive or so-called "female cell," called an oosphere.

The product of the union of an antherozoid and an oosphere is termed an oospore.

In many cases the differentiation of the sexual cells does not proceed so far as the formation of antherozoids or of distinct oospheres; these cases I shall investigate with the others in detail presently.

The antheridia are stalked sacs, with a single wall of cells, and the spiral antherozoids arise by free-cell formation from the cells of the interior.

Down this canal pass one or more antherozoids, which become absorbed into the oosphere, and this then secretes a wall, and from it grows the second or asexual generation.

Thus in the Rhizocarpæ there are two kinds of spores, microspores and macrospores, producing prothallia which bear respectively antheridia and archegonia; in the Lycopodiaceæ, the two kinds of spores produce very rudimentary prothallia; in the cycads and conifers, the microspore or pollen grain only divides once or twice, just indicating a prothallium, and no antheridia or antherozoids are formed.

The upper part of the antheridium becomes separated from the parent stem by a septum, and its contents are converted into ciliated motile antherozoids.

The adjacent sporangia also become cut off by septa, and the investing membrane, when mature, opens: it a beak-like prolongation, thus permitting the inclosed densely congregated green granules to be penetrated by the antherozoids which swarm from the antheridium at the same time.