THALLOPHYTES 
walls of the two neighboring cells are observed to bulge out. The fila- 
ment does not grow indefinitely in length, but breaks up now and then 
by the disorganization of one or more cells, and 
each fragment begins to construct another colony. 
This process of fragmentation results in the 
multiplication of colonies, which may be called 
colonization. 
The protoplast of Oscillatoria exhibits the two 
regions described under Gloeocapsa for Cyano- 
phyceae in general; in fact, this differentiation is 
probably more evident in Oscillatoria than in any 
other common form. The most striking feature 
of the plant, however, is the characteristic sway- 
ing and revolving movement of the filaments, a 
movement which suggested the name. If a mass 
of filaments be placed on a solid substratum, the 
filaments begin a creeping movement and become 
spread out radiately in a film. It is evident that 
this movement is possible only as the cells of a 
filament work together, and this introduces into a 
colony of cells the idea of an individual composed the ceUs of the simple 
of many cells. In fact, the many-celled colony f lam " nt f s sh the dif " 
' J J ferentiation of the proto- 
merges so gradually into the many-celled individual p i as t into the peripheral 
that there is no boundary between the two. pigment region and the 
Kr . T ii.- t iU i i central region; in the 
Nostoc. In this form the colony is also a , ., , ' ., 
1 center of the latter there 
filament, but when the cells divide, they so nearly appears an irregular ag- 
separate and round off that they become tangent gregation of dark ma- 
,1 ,,. . ,-1 11. terial (chromatin) char- 
to one another, resulting in a filament resembling acteri ; tic of a nu ; leus . 
a string of beads. Each filament has its own 
mucilaginous sheath, as in Lyngbya, but there is an extraordinary 
development of mucilage in connection with groups of filaments. As 
a consequence, Nostoc appears in nature as lumps of jelly, in which 
numerous filaments are found embedded (figs. 7, 8). 
The most noticeable fact in reference to these filaments is that the 
cells are not all alike. At intervals cells appear which differ in contents 
and usually in size from the ordinary working cells. They are derived 
from ordinary working cells, which usually enlarge, lose their contents, 
and become thick-walled. The loss of pigments makes these cells 
stand out very distinctly in the filament They are called heterocysts, but 
