The highest form of the sexual reproduction is the =Egg- or Oogamous= fertilisation. The two coalescing cells are in the main unlike each other in form as well as size. The one which is considered as the male, and is known as the _spermatozoid_ (_antherozoid_), developes as a rule in large numbers in each mother-cell (_antheridium_); they are often self-motile (except in the Florideæ, where they are named _spermatia_), and are many times smaller than the other kind, the female, which is known as the _egg-cell_, (_oosphere_). The egg-cell is always a motionless, spherical, primordial cell which can either float about freely in the water, as in the Fucaceæ (Fig. 9), or is surrounded by a cell-wall (_oogonium_); generally only one oosphere is to be found in each oogonium, but several occur in _Sphæroplea_ (Fig. 10). The result of the spermatozoid coalescing with the egg-cell is, as in this case, the formation of a oospore, which generally undergoes a period of rest before germination (the Florideæ are an exception, a fruit-body, _cystocarp_, being produced as the result of coalescence). An example of fertilisation is afforded by the Alga, _Sphæroplea annulina_ (Fig. 10). The filamentous thallus is formed of cylindrical cells with many vacuoles (_r_ in _A_); some cells develope egg-cells (_B_), others spermatozoids (_C_), the latter in a particularly large number. The egg-cells are spherical, the spermatozoids of a club- or elongated pear-shape with two cilia at the front end (_G_; _E_ is however a swarmspore). The spermatozoids escape from their cells through apertures in the wall (_o_ in _C_) and enter through similar apertures (_o_ in _B_) to the egg-cells. The colourless front end of the spermatozoid is united at first with the “receptive spot” of the egg-cell (see _F_), and afterwards completely coalesces with it. The result is the formation of a oospore with wart-like excrescences (_D_). The female (_parthenogenesis_) or male (_androgenesis_) sexual cell may, sometimes without any preceding fertilisation, form a new individual (_e.g. Ulothrix zonata_, _Cylindrocapsa_, etc.). =Systematic division of the Algæ.= The Algæ are divided into the following ten classes: 1. SYNGENETICÆ; 2. DINOFLAGELLATA, or PERIDINEA; 3. DIATOMACEÆ; 4. SCHIZOPHYTA, FISSION-ALGÆ; 5. CONJUGATÆ; 6. CHLOROPHYCEÆ, GREEN-ALGÆ; 7. CHARACEÆ, STONE-WORTS; 8. PHÆOPHYCEÆ; 9. DICTYOTALES; 10. RHODOPHYCEÆ. Among the lowest forms of the Algæ, the Syngeneticæ, the Dinoflagellata, and the unicellular Volvocaceæ (Chlamydomoneæ), distinct transitional forms are found approaching the animal kingdom, which can be grouped as animals or plants according to their method of taking food or other characteristics. Only an artificial boundary can therefore be drawn between the animal and vegetable kingdoms. In the following pages only those forms which possess _chromatophores_, and have _no mouth_, will be considered as Algæ.