distinguished with the naked eye. It looks not unlike a heart-shaped mass of greenish tissue quite flat on the ground, and is called a _thallus_. Up to this point, then, we may trace the story of any fern which has thrown off its cloud of spores and from which develops this tiny thallus, looking not in the least like a fern nor as though it could ever be modified into one. Because this thallus is, [Illustration: FIG. 76.--PROTHALLUS OF A FERN SEEN FROM THE UNDER SIDE (_A_^{1}) archegonia, (_A_^{2}) antheridia, and (_A_^{3}) the rhizoids. _B_: Prothallus, showing the young plant with its first leaf (_B_^{1}), its own roots (_B_^{3}) and the rhizoids of the prothallus (_B_^{2}). Drawing and legend for it slightly altered from Kraemer.] in the truest sense, merely a preparation for the process that _will_ produce another fern, it is always known as a _prothallus_. The prothallus is thus the first stage in the reproduction of ferns, a very simple stage, with only the faintest indication that the thallus might be considered the vegetative and its rhizoids perhaps the rootlike counterparts of foliage and roots of mature ferns. As we shall see presently, even this differentiation has not the significance that such a structure in flowering plants would indicate. There is not, as yet, the faintest indication of sexes that need to mate in order to produce their young. The spore has so far only produced a tiny flat body of green tissues with a few rootlike threads, so unlike the fern from which it started that its true significance, or even the fact that it had ought to do with ferns was not known until about the middle of the last century.