Dr. C. A. Kofoid on Pleodorina illinoisensis. 145 



many cells a faintly marked notch or furrow (fit., fig. 1) is to 

 be detected on one side of the ehromatophore at the anterior 

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

 the sides of the ehromatophore 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. VI. figs. 7, 8), but in 

 the cells of the young colony it again occupies a central 

 position. The ehromatophore 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.) t which occupies a lateral 

 position in the ehromatophore, 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 ehromatophore, and may occur in any 

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

 Shaw (1894) 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 //,, 

 and in the older colonies of 2"5/u.. 



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

 midst of a mass of protoplasm enclosed by the ehromatophore. 

 In mature gonidial cells before division it has a diameter of 

 7-8 /a, and contains a sub-central nucleolus (jwl.) whose 

 diameter is 3 fM. The nucleolus stains deeply with picro- 

 carmine, and is by this means easily distinguished from the 

 pyrenoids, which it resembles in appearance and size. The 

 nuclear membrane is detected with difficulty. It encloses a 

 faintly stained nuclear reticulum (r.). In the younger cells 

 the nucleus is much smaller (4-5 (i), the nucleolus is rela- 

 tively larger, and the reticulum is not evident. In the living- 

 cell the nucleolus alone can be seen in the midst of the 

 greyish protoplasmic mass at the centre of the cell. The 

 protoplasm is continued from this central region peripherally, 

 in the axis of the cell as a slender column (/>.c), to the 

 anterior end, where it includes the stigma and bases of the 

 two flagella. A protoplasmic mantle enclosing the ehromato- 

 phore was not demonstrated. 



The stigma or eye-spot (s.) lies at the anterior end of the 



cell, near its axis, and is often so placed that an equilateral 



triangle may be drawn with it and the bases of the two flagella 



as apices. It is of a bright reddish-brown colour, though in 



Ann, & Mag. N. Hist. Ser. 7. Vol. vi. 10 



