380 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
gelatinous envelope becomes stained with haematoxylin and also with tannate vesuvine. With 
the latter it is homogeneous and shows no prismatic structure characteristic for such coverings in 
many similar forms of alge. Two layers can often be detected, however, the inner one the more 
dense. The outer one probably belongs to the preceding generation (fig. vu, 4). 
There is but a single, vivid green, homogeneous. parietal chloroplast, through which there is 
an opening at one side or near one end. From this opening the chloroplast gradually increases in 
thickness toward the opposite side, where it incloses a large pyrenoid (fig. vil, 1, 2). The 
central portion of the pyrenoid shows a crystalline character, and is surrounded by a thick starch 
envelope. The single spherical or elongated nucleus lies near the center (fig. vir, 1a). With 
hematoxylin a network throughout the cell is brought to view, and is probably due to the arrange- 
ment of protoplasm and vacuoles. Oil is found in greater or less quantity in all cells. This oil is 
turned brown by osmic acid and becomes dissolved in absolute alcohol. 
Reproduction occurs by means of bipartition or repeated bipartition of the cell contents, so 
that two, four, or eight cells are formed from one individual. The first division is a transverse 
one (fig. vil, 3a); then, if the process continues, the next divisions, dividing the products of 
the first, take place at right angles to the first division and at right angles to each other (fig. V1, 
3b). Each part then becomes invested with a membrane and the outer gelatinous substance 
begins to be excreted by the cells before they are set free from the mother cell. The enveloping 
mother membrane becomes much distended, probably by means of the gelatinous substance (fig. 
vu, 4), until finally it becomes ruptured at one point and the two, four, or eight cells are set free, 
leaving the remnant of the old cell wall clinging to the outer surface of the gelatinous covering 
(fig. vit, 3, 5). Though in young individuals there is but a single chromatophore, this very 
soon becomes divided into two or four, long before the division of the cells occurs, so that in a 
culture the great majority of cells contain four chromatophores, and it was first thought that four 
was the normal number. Preceding the division of the chlorophyl body occurs the division of the 
pyrenoid. The division of the nucleus does not occur until just before the formation of the 
daughter cells and long after the division of the chloroplast. 
Physiology.—Though in the natural conditions the cells are usually found united into larger 
or smaller complexes, the aggregated form of growth is by no means necessary to existence and is 
characteristic only under certain conditions, for under the various environments to which the alga 
was submitted in artificial culture, it was found that either the isolated or aggregated condition of 
the cells could be produced at will. Cultures were made in various media, such as different 
concentrations of Knop’s solution, decoctions of earth, and solutions containing animal and 
vegetable matter taken in the tow from the lake and stagnant ponds. Of the various solutions 
used the organic solutions seemed best to reproduce the aggregated form as it is found in the 
plankton, but even here the masses were not quite as large as those in the natural condition, 
although the appearance of the individual cell was perfect (fig. vi1, 4). The concentration of the 
organic solution had a marked effect in producing the isolated or aggregated condition in the 
development. The exact amount of substance in solution was not determined, but it was found 
that in the solution which was taken as a standard the cells were all grouped in colonies, and 
several of these were united into compound colonies. In the same solution, but of one-half the 
standard concentration, and even in the above solution after the concentration had become 
reduced by the growth and increase of the algae, only isolated cells were formed, which were 
distributed throughout the liquid instead of resting on the bottom of the culture glass, as occurred 
in most cultures. Each individual cell was surrounded by a gelatinous covering one-half to 
twice the diameter of the cell in thickness, but these were not held together by a common 
envelope (fig. vu, 5). The vigor of the culture, however, seemed just as great as where the 
families were formed. It is probable that the greater amount of water present in proportion to 
the organic matter reduced the consistency of the gelatinous substance, and the connection 
between the cells was broken. 
Knop’s solution of different concentrations did not seem to be favorable to development, for in 
no case was the appearance normal. Cells were usually isolated and a great amount of oil was 
developed in the contents. In 1 per cent and 0.4 per cent increase was slight, while in 0.1 per cent 
and in 0.05 per cent, notwithstanding a very large amount of oil being present (fig. vi1, 5), increase 
was rapid. In the decoction of earth growth was abundant and normal, except for the presence 
of a large amount of oil. 
