372 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
organisms, causing them at will to reproduce, or to assume any stage in their 
development which we may desire. 
At present great confusion exists in the nomenclature of these lower vegetable 
organisms. Many, in certain stages of their development, can not be distinguished 
from one another, and even some polymorphie filamentous algee have often been 
confused with unicellular forms, as they may assume a unicellular condition in 
which even a skilled observer is unable to distinguish them with certainty from true 
unicellular plants. The entire life history should therefore be traced, and although 
there may be stages in the development of different ones which can not be readily 
distinguished, a broader knowledge must aid in recognition. 
To determine accurately the life history of a species, observation should be 
made from pure cultures. The need of this has been pointed out by Klebs 796, 
Artari 792 and Senn °99, and has often been suggested by the inaccurate work of 
a number of investigators, in which many species have been confused. If one 
start with a single cell or a small cluster of cells, all of which are known to be the 
same, and from these procure an unlimited supply of absolutely pure material, then 
one can assert with certitude that whatever developments occur they are character- 
istic of that species, whereas if the material be not pure one is easily misled as to 
the connection of different forms. But even a pure culture under one set of condi- 
tions is not sufficient. Material should be subjected to all possible conditions which 
might ever occur in nature, and the effect of these conditions studied in all phases 
of development. When this is done we may venture to classify the organism, and 
then many phenomena not now understood will-probably be explained. 
In the present study, which was continued during the summer of 1898, 1899, 
and 1900 at Put-in Bay, Lake Erie, and at Ann Arbor, Mich., in 1900, the work 
has been largely preparatory, and has been confined to comparatively few of the 
numerous forms present. The first summer was devoted principally to becoming 
acquainted with the forms found in Lake Erie, and in experimenting with culture 
media, that pure cultures of the different forms might be obtained and the condi- 
tions governing development be determined. It was soon found that although alge 
existed side by side in the water of the lake, the conditions which determined their 
growth were not the same—that the favorable conditions for development must be 
determined for each genus, and often for each species individually. Comparatively 
few would live at all in the media which are so generally used for more hardy forms 
found in stagnant pools. After determining the favorable media for some of the 
most common forms in the plankton, the following summers were devoted to tracing 
their life history and studying such biological facts as could be determined. 
As the amount of work to be done was so great, it was thought best to limit 
investigation to some special group, and as the Chlorophycew are more easily main- 
tained in culture and are more varied in their development, requiring more constant 
observation, they were taken first. All species to which special attention is given 
in this paper, unless a statement to the contrary is made, were taken from the 
plankton, and an abundance of pure material was obtained by cultivation. Cultures 
were made in the ordinary Stender dishes, and parallel with these were also 
continued hanging drop cultures, where the development of the same individuals 
could be observed from day to day and no step be overlooked. 
Aside from tracing the development of a number of the members of this group, 
