ThA aE, by PRU Fler a fief 
The Cell as a Memter of a Group. 37 
Certain differences exist in the groups of similar cells, 
according to which they: are distinguished as families or 
colonies, tissues, and fusions of cells. 
A cell-family originates from a single mother-cell ; and 
each of the daughter-cells preserves its own individuality, 
which can even proceed as far 
as separation from the family. 
In Pediastrum rotutla, for ex- 
ample (Fig. 53), the “sreen 
contents of each mother-cell 
break up into several spherical 
F See Pe ee aoe daughter-cells ; the innermost 
cells. (x 250.) membrane of each separate 
mother-cell then protrudes through a fissure and rapidly en- 
larges, in order to make room, in the cells formed in it, for a 
creeping motion which lasts for some time. As soon as_ 
_ Fic. 59.—Pleurococcus superbus: aa family of four cells; the cells themselves are 
naked, but the whole family is enclosed’in a common cell-wall consisting of six 
layers ; 6 each cell has developed a separate cell-wall; c the original cell-walls 
‘. ‘have burst, and the separate cells have become free ; they continue for some time 
in motion, then come to rest, and develope new families. (After Cienkowski, x 320.) 
this motion ceases, the sister-cells place themselves together 
so as to form a plate, and produce in their development a 
-new colony. Cell-families occur only in the lowest classes 
of plants, the Algz and Fungi (Fig. 59). 
The union of cells into ¢sswes and cell-fusions 18 perma- 
nent, and is combined with a limitation in various ways of the . 
