Algae and the Principles of their Evolution . 105 
Sphaerella have been described by several authors as new 
species of Chlamydomonas 1 ( Chi . nivalis , Lagh., Chi. sanguined ). 
In some forms of Chlamydomonas the cell- walls become more 
and more reduced and are sometimes altogether wanting. From 
these forms to the metabolic species and genera 2 , such as 
Chloraster , Pyraminomonas , and P olyhlepharis , the passage 
is easy. 
The genus Gonium , which has been separated in nearly all 
the systems from Chlamydomonas and placed in a different 
family, is very closely related to the latter. They may be 
considered as Volvocineae in which the tetrasporoid-stage 
is undergone in a motile condition. Gonium sociale can be 
very easily reduced to a unicellular form which can multiply 
in this stage for a certain time, as is shown by our pure 
culture experiments. A Gloeocystis-condition is also very 
easily obtained by gradual concentration of the nutritive 
solution. G. pectorale , which at its normal stage is a sixteen- 
celled motile colony, has been reduced in our experiments 
to four-celled colonies, which are very like G. sociale. We 
could only determine by further cultivation to which of the 
two species such forms belonged. In these young stages 
of G. pectorale each cell of the four-celled colony divides into 
sixteen, and thus produces successively or simultaneously four 
new sixteen-celled colonies. This can be very clearly verified 
in cases where only a single cell of the tetrad divides. This 
species is also capable of subsisting in a unicellular stage. 
In Pandorina the earliest stage is the larval condition , which 
is transformed by degrees into a globular condition by the 
rolling in of the edges ; in certain cultures of this species, 
however, I could obtain motile stages of the shape of Gonium 
pectorale 3 , only less regular ; and also by gelification, resting- 
stages like the Gloeocystis- stage of Gonium sociale. 
1 Lagerheim, Die Schneeflora des Pichincha, Berichte der Deutsch. Bot. Ges. 
1892, p. 517, tab. xxviii. 
2 See Dill, Die Chlamydomonas und Verwandte, Jahrbiicher fiir wissensch. Bot. 
xxviii, 1895. 
3 Loc. cit., Materiaux pour servir, &c., I, pp. 593 and 597. 
