Algae and the Principles of their Evolution . 1 1 7 
formations I have obtained in my pure cultures, and have 
also observed them in their natural habitats on many occasions. 
Naegeli has distinguished under the name of Cystococcus 
a unicellular Alga which he believed quite different from 
Pleurococcus. In his drawings the cells are globular, with 
a stellate chromatophore, and a very distinct pyrenoid, which 
has often been taken for the nucleus. On one side there 
is a clear space containing a nucleus. He also describes 
a cell-division in which the numerous daughter-cells are 
polyhedric by mutual pressure. It is well known that Cysto- 
coccus can produce biciliate zoospores. I have also observed 
very minute gametes fusing together a short time after their 
expulsion. This plant, if only known in this stage, could be 
taken as a type of the Protococcoideae, for in it may be very 
easily observed the transformation of the zoospores into non- 
motile spores. But Cystococcus is not a member of the Proto- 
coccoideae at all ; it is merely a stage of the development 
of Pleurococcus. 
I have already stated how in the same Pleurococcus the 
different cells can produce a globular sporangium with spores, 
a transformation of a part of the thallus into a false spo- 
rangium, and branched or unbranched filaments, and the 
same phases may be observed in the Cystococcus in its 
typical form, and the true Pleurococcus , which is provided 
with very different chromatophores. For instance, I have 
repeatedly noticed on the same thallus, with or without 
pyrenoids or filaments, typical Pleurococcus- cells with parietal 
chromatophores, and one or more Cystococcus- cells with 
a more or less stellate chromatophore. In such cases 
I have seen the expulsion of the zoospores or gametes, 
whilst in other cells of the same thallus spores were formed l . 
1 There are certainly two varieties or species known under the name of 
Pleurococcus vulgaris ; the first with more or less stellate chromatophores and 
pyrenoids, which in its Cystococcus-stage can produce motile spores ( Pleurococcus 
vttlgaris , Menegh. p. p., non Naeg.), and the other without pyrenoids and with 
less stellate chromatophores, from which I could not obtain motile elements 
(PI. vulgaris , Naeg. p. p., non Menegh.). In my cultures the two remained quite 
distinct. 
