This green cushiony prothallus keeps on growing, its heart-shaped mass becoming divided into an obviously left and right hand side and the rhizoids multiplying in number. They are always borne on the lower side next the ground, or next whatever the prothallus may be growing on. Near the notch of the heart-shaped prothallus are developed a few flask-shaped bodies which contain within them an egg cell or single ovum, the female reproductive body. By a series of changes this egg cell becomes embedded in a mucilaginous material. This flask-shaped body with the female egg cell inside is known as the _archegonium_. From among the rhizoids there may, at about the same time, be found developing small globular organs that have in them a number of tiny cells, each of which has attached many minute threadlike tails. The globular organs, with their minute, tailed cells are known as _antheridia_, and comprise the male reproductive equipment. Just as in flowering plants, neither the _archegonia_ (female) nor the _antheridia_ (male) can produce offspring without mating and the method by which this marriage is accomplished differs tremendously both in practice and in its significations from that in phanerogams. In the first place, the male and female reproductive cells are separated by a considerable distance, they are both inclosed in structurally different casings, and the whole operation is so microscopic that insects can be of no service. Nor can the wind do for them what we have seen that it does for the pollen of pines and grasses. Of the aids to fertilization there remains then only the water, which plays such an important part in the mating of the eelgrass and ditch grass among flowering plants. But in these ferns a very different drama is about to be enacted. The male cells, as we have seen, are provided with slender tails, which are movable. They move, in fact, to such good purpose that the male cell can actually swim in the water. Of course its minute size demands only the merest drop of water, in which it will take the only excursion of its brief life. For just as soon as it is mature, a heavy dew or the tiniest particle of water will set free the little male messengers. The water too has not been without effect on the female cell. More remarkable still, this mucilaginous matter contains in it a substance that acts as a lure to the swimming male cells. In any event they do swim directly to the entrance of the female cell’s abode, through it and to her, when the union is effected. At once there is thrown across the entrance a membrane that excludes all other males, and the fertilization is complete. From this union of the male and female cells a true young fern begins to develop. First a young leaf and roots, finally a stem and in the end, of course, a full-grown fern producing spores, ready to renew the whole process.