On Pleodorina illitioisensis. 279 



tint with long-continued action of iodine and sulphuric acid. 

 In the case of diseased colonies hereafter mentioned the cell 

 membranes persist, often retaining their original form and 

 shape, after the' entire disappearance of the contents. 



The greater part of the cell contents consists of what seems to 

 be one large chromatophore (PL XXXVI., Fig. 2, chr.), which 

 occupies all of the cell except the centrally placed nucleus 

 with its enveloping protoplasm, and a slender column {p. c.) 

 passing from this region to the anterior end of the cell. In 

 many cells a faintly marked notch or furrow (fu., Fig. 1) is to 

 be detected on one side of the chromatophore at the anterior 

 end of the cell. This seems to mark the line of contact of 

 the sides of the chromatophore which has surrounded the 

 nucleus. In the 2- and 4-cell stages of the gonidial cells 

 the nucleus and the protoplasmic mass are plainly seen 

 to occupy one side of the cell (PI. XXXVII., Fig/ 7, 8), 

 but in the cells of the young colony it again occupies a central 

 position. The chromatophore is uniformly of a bright chloro- 

 phyll-green, and shows a finely granular structure under high 

 magnification. In the youngest colonies each cell contains 

 but a single spherical pyrenoid (pr.), which occupies a lateral 

 position in the chromatophore, in the inner hemisphere of the 

 cell. In the older colonies the number of pyrenoids increases, 

 as many as twelve having been found. They are scattered 

 irregularly through the chromatophore, and may occur in 

 any part of it. A similar increase of pyrenoids is reported 

 by Shaw ('94) for P. californica. In the vegetative cells the 

 number of pyrenoids is often but 2-4, and is, as a rule, less 

 than that of the gonidial cells. In a very few instances as 

 many as eight have been found, and in one old colony the 

 vegetative cells seemed to be packed full of pyrenoids. In the 

 young colonies the pyrenoids have a diameter of about 1 m, 

 and in the older colonies of 2.5 /'. 



The nucleus (n.) lies in about the center of the cell in the 

 midst of a mass of protoplasm enclosed by the chromatophore. 

 In mature gonidial cells before division it has a diameter of 

 7-8 M, and contains a sub-central nucleolus (ncl.) whose 

 diameter is 3 /<. The nucleolus stains deeply with picro- 



