274 Phillips on a Comparative Studyjof the 
chromatin, reserve products, assimilation products, etc. 
The peripheral zone is looked upon as a chromatophore by 
some, while others consider it as the general cytoplasm, 
throughout which the coloring matter is diffused. The wall 
is composed of a substance unnamed as yet, but supposed to 
be a modification of cellulose, probably quite close to fungus- 
cellulose. It was in the hope of adding something to our 
knowledge of these organisms, which might aid in the solu- 
tion of these cytological problems that the present work was 
undertaken. 
Material Used. 
The choice of organisms for this investigation was gov- 
erned by several considerations: (1) The availability of 
material. The greenhouses and Botanical Garden of the 
University of Pennsylvania have furnished excellent places 
for the collection and culture of these organisms, though 
the collections were by no means confined to these localities. 
(2) Plants were selected primarily with a view to the large 
size of their cells. Those forms in which the cell contents 
were especially masked for any reason, or where the cells 
were very small, were discarded, and the larger and clearer 
organisms used in preference. (3) Several different organ- 
isms were employed in order to avoid the mistaking of arti- 
facts for natural conditions, and to eliminate the “individual 
factor” as much as possible. 
Though the work here reported deals mainly with the 
species of Oscillaria, Nostoc and Cylindrospermum investi- 
gated, the results with the other organisms were essentially 
the same, and to speak of each organism in detail would be 
but reiteration. Therefore the morphological structures 
have been made the basis of report, they being so constant, 
varying only with the forms of the cells of the different 
plants, that they may be considered as belonging to all of 
the forms except where otherwise noted. The plants studied 
were as follows: Nostoc (three species), Nostoc in Collema, 
