There are several other groups of algae that confirm and illustrate in various particulars the principles of sexual evolution that we have traced in the three lines of development here described. The Siphonales duplicate the history in most of the stages. Isogamy with variation in the size of the gametes is found in several of the remaining groups. Heterogamy in its most extreme form is presented in such isolated types as the stoneworts (Charales), Oedogonium, Bulbochaete and Sphaeroplea. All these types stand as representatives of lines of extinct ancestry, whose sexual evolution must have passed through the stages that we have described.
The red algae (Rhodophyceae) do not enter into this discussion. They started with sexuality at an advanced stage of heterogamy.
Let us now briefly summarize with examples the steps in sexual evolution which we have discussed, beginning with isogamy, at the dawning of sex, and ending in heterogamy.
First Stage.—Isogamy with exactly similar gametes; the condition at the origin of sex. Exemplified by many of the lower algae, Hydrodictyon, Ulotlirix, Viva, Cladophora, etc., certain species of the lower brown algae and unicellular green.
Second Stage.—Isogamy with gametes similar in form but of different sizes, the female large and richly nourished, the male relatively small. Illustrated by species of Chlamydomonas and Ectocarpus, Bryopsis and the forms that also illustrate the third stage. An index to this condition is the differentiation of the gametangia with respect to the number of gametes developed. The female gametangium tends to reduce the number until a single egg takes all the material of the oogonium. The male gametangium increases the number of sexual products, becoming an antheridium that may develop numerous sperms.
Third Stage.—Isogamy in that peculiar form when the gametes are similar in form at the time of their discharge from the gametangia, but different at the time of fusion, because the female gamete becomes a motionless cell. Examples: Ectocarpus siliculosus and secundus and Cutleria multifida. This stage is the transition point between isogamy and heterogamy. Morphologically these gametes are isogamous; physiologically they are heterogamous.
Fourth Stage.—Heterogamy which has several grades in the degree of differentiation and specialization of the egg and sperm.
Fifth Stage.—The retention of the egg in the oogonium (female gametangium), a condition peculiar to but not at all universal among heterogamous higher algae. Illustrated by Oedogonium, Bulbochaete, Coleochaete, Sphaeroplea, Chara and Vaucheria. This stage would be developed quickly when once started, and a tendency in this direction is probably shown in Aphanochaete.
