Algae and the Principles of their Evolution . 109 
cases very clearly shown, partly by experiment, partly by 
comparison with allied species. 
Sometimes the autospores are after their extrusion quite 
free ( Lager heimia , Oocystis p. p., Dactylococcus), sometimes 
they are surrounded by the gelatinous envelope formed as 
stated above ; this is the case in Kirchneriella , Nephrocy tiu m , 
&c. Lastly, the autospores which are not in this case 
surrounded with this jelly, are united together by threads 
formed of this substance, as in Dactylococcus infusionum h 
It will be easily understood that under such conditions the 
arrangement is very variable, the cells being attached together 
either in one continuous row or in radiating threads, or 
forming a branched gelatinous filament. 
In this case the form of the mother-cell is susceptible of 
great variation (in the same species), the more modified form 
being a spindle-shaped cell. All these forms maybe obtained 
in a pure culture at the same time by gradual transformation. 
When the four spindle-shaped autospores are not set free 
by the solution of the gelatinous matter, they often remain 
attached in a single row as they were in the mother-cell, and 
by their expansion in a plane constitute the well-known 
coenobium of Scenedesmus acutus. All these transformations, 
and those above described, I have observed a great many 
times, and it is very easy to repeat them. 
As a further step, this little coenobium producing similar 
autospores united together in a row at the instant of their 
expulsion, the production of an auto-colony takes place. 
Under commonly observed conditions this so often occurs 
that it has been considered as the only mode of reproduction 
in Scenedesmus. It becomes more constant, however, in some 
other species of the same genus, such as S. quadricauda , 
which is generally provided with long spines. In Raphidium 1 2 
we have in a way a repetition of this method of propagation ; 
the long spindle-shaped cell divides within the thin mother- 
1 Chodat et Malinesco, Sur le polymorphisme du Scenedesmus acutus , Bull. Herb. 
Boiss., 1896, t. iv, p. 184; et ibid , 1893, p. 640 ; et Chodat, Materiaux, I, pp. 602, 608. 
■ 2 Chodat, Materiaux, I, p. 608, tab. 26. 
