The simplest and lowest form is termed =conjugation=, or =isogamous= fertilisation, and is characterized by the fact that the two coalescing cells (termed gametes) are equal, or almost equal, in shape and size (the female gamete in the _Cutleriaceæ_, _e.g. Zanardinia collaris_, Fig. 7, is considerably larger than the male gamete). The cell in which the _gametes_ are developed is called a _gametaugium_, and the reproductive cell formed by their union--which generally has a thick wall and only germinates after a short period of rest--is termed a _zygote_ or _zygospore_. The conjugation takes place in two ways:-- (_a_) In the one way the gametes are motile cells (_planogametes_, _zoogametes_, Fig. 8), which unite in pairs during their swarming hither and thither in the water; during this process they lie side by side (Fig. 8 _d_), generally at first touching at the clear anterior end, and after a time they coalesce and become a motionless _zygote_, which surrounds itself with a cell-wall (Fig. 8 _e_). This form of conjugation is found in _Ulothrix_ (Fig. 8 _d_), _Acetabularia_, and other Algæ (Figs. 45, 56, 66). [Illustration: FIG. 8.--_Ulothrix zonata_: a portion of a thread with zoospores, of which two are formed in each cell (zoosporangium), the dark spots upon them are the “red eye-spots”; 1, 2, 3, 4 depict successive stages in the development of the zoospores; _b_ a single zoospore, at _v_ the pulsating vacuole; _c_ portion of a thread with gametes, of which sixteen are formed in each gametangium; _d_ gametes free and in conjugation; _e_ conjugation has been effected, and the formed zygotes are in the resting condition.] (_b_) Among other Algæ (_e.g. Diatomaceæ_ and _Conjugatæ_), the conjugating cells continue to be surrounded by the cell-wall of the mother-cell (_aplanogametes_ in an _aplanogametangium_); the aplanogametangia generally grow out into short branches, which lie close together and touch one another, the wall at the point of contact is then dissolved (Fig. 39). Through the aperture thus formed, the aplanogametes unite, as in the first instance, and form a rounded zygote, which immediately surrounds itself with a cell-wall. Various modifications occur; compare Figs. 37, 39, 41, 43. [Illustration: FIG. 9.--Fertilisation in the Bladder-wrack (_Fucus vesiculosus_).] [Illustration: FIG. 10.--_Sphæroplea annulina._]