Fertilisation takes place in the following way. The oogonium is a large ellipsoidal, swollen cell (_og_, in Fig. 54 _A_), whose contents are rounded off into an oosphere with a colourless receptive-spot (see _B_); an aperture is formed in the wall of the oogonium, through which the spermatozoids are enabled to enter (_B_). The spermatozoids are produced either directly, as in _D_ (in pairs), in basal cells of the filament, or indirectly. In the latter case a swarmspore (_androspore_) is formed which comes to rest, attaches itself to an oogonium, germinates, and gives rise to a filament of a very few cells--_dwarf-male_ (_A_, _B_, _m_). The spermatozoids are formed in the upper cell of the dwarf-male (_m_), and are set free by the summit of the antheridium lifting off like a lid. On the germination of the oospore (_C_), which takes place in the following spring, 4 zoospores are produced (_F_) (_i.e._ the sexual generation); these swarm about for a time, and ultimately grow into new filaments. [Illustration: FIG. 55.--_Coleochæte pulvinata. A_ A portion of a thallus with organs of reproduction; _a_ oogonium before, _b_ after fertilisation; _c_ an antheridium, closed; _d_ open, with emerging spermatozoid. _B_ Ripe oogonium, with envelope. _C_ Germination of the oospore. _D_ Zoospore. _E_ Spermatozoid.] Order 7. =Coleochætaceæ.= The thallus is always attached, and of a disc- or cushion-shape, formed by the dichotomous branching of filaments of cells united in a pseudo-parenchymatous manner. Each cell has only one nucleus. Asexual reproduction by zoospores with 2 cilia (Fig. 55 _D_), which may arise in all the cells. Sexual reproduction by oogamous fertilisation. The spermatozoids resemble the swarmspores, but are smaller (_E_), and originate singly (in the species figured) in small conical cells (_c_, _d_ in _A_). The oogonia are developed at the extremities of certain branches: they are bottle-shaped cells with very long and thin necks (_trichogyne_), open at the end (_a_ in _A_); at the base of each oogonium is a spherical oosphere. The spermatozoids reach the oosphere through the trichogyne, or through an aperture in the wall when the trichogyne is absent, and fertilisation having taken place, the oogonium becomes surrounded by a cell-layer (envelope), which grows out from the cells near its base (_b_ in _A_), and in this way a kind of fruit is formed (_B_) (_spermocarp_, _cystocarp_). The oospore, next spring, divides and forms a parenchymatous tissue (homologous with the Moss-sporophyte); this bursts open the envelope (_C_), and a zoospore (homologous with the spores of the Moss-capsule) arises in each of the cells, and produces a new _Coleochæte_. We have then, in this case, a still more distinct alternation of generations than in _Œdogonium_. Only one genus, _Coleochæte_, is known, but it contains several species, all living in fresh water.