It is clearly quite illogical to regard the chromacea as a class or family of the algæ, as is still done in most manuals of botany. The real algæ--excluding the unicellular diatomes and paulotomes, which belong to the protophyta--are multicellular plants that form a _thallus_ or bed of a certain form and characteristic tissue. The chromacea, which have not advanced as far as the real nucleated cell, are unnucleated cytodes of a lower and earlier stage of plant-life. If one would compare the chromacea with algæ or other plants at all, the comparison cannot be with their constituent cells, but merely with the chromatophora or chromatella, which are found in all green plant-cells, and form _part_ of their contents. To be more precise, these green granules of chlorophyll must be regarded as organella of the plant-cell, or separated plasma-formations which arise beside the nucleus in the cytoplasm. In the embryonic cells of the germs of plants and in their vegetation points the chromatophora are as yet colorless, and are developed, as solid, very refractive, globular, or roundish granules, from the firm layer of plasm which immediately surrounds the nucleus. Afterwards they are converted, by a chemical process, into the green chlorophyll granules or chloroplasts, which have the most important function in the plasmodomism or carbon-assimilation of the plant. The fact that the green chlorophyll granules grow independently within the living plant-cell and multiply by segmentation is very important and interesting. The globular chloroplasts are constricted in the middle, and split into two equal daughter-globules. These daughter-plastids grow, and multiply in turn in the same way. Hence they behave within the plant-cell just like the free-living chromacea in the water. On the strength of this significant comparison, one of our ablest and most open-minded scientists, Fritz Müller-Desterro, of Brazil, pointed out in 1893 that we may see in every green vegetal cell a symbiosis between plasmodomous green and plasmophagous not-green companions (_cf._ my _Anthropogeny_, figs. 277 and 278, and in the text).