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



under observation the cells appeared normal, showing no trace 

 of degeneration or division. In one instance only have I 

 found a specimen in which the vegetative cells had divided 

 beyond question. This was a colony in which the gonidia 

 had completed their division but had not escaped. Three of 

 the vegetative cells were in the two-cell stage and one was 

 undivided. In two instances matured colonies have been 

 found in which four smaller daughter colonies (of eight and 

 sixteen cells respectively) were present at one pole. 



The gonidial cells (PI. V. fig. 3) constitute the remainder 

 of the colony. They usually number 28, rarely 12 or 60, 

 and occupy the parts behind the anterior polar circle of vege- 

 tative cells. These cells in most instances can be easily 

 distinguished by their larger size. In form they are spherical, 

 though some specimens in preserved material are slightly 

 flattened on their inner ends. In diameter they range in 

 seemingly full-grown colonies from 15 to 25 /x, averaging in 

 twelve specimens 19'2 fx. Their dimensions just before their 

 division, that is, in colonies in which division has begun, also 

 show the extreme range quoted above, the smaller diameter 

 having been found in a sixteen-cell colony. As a rule the 

 gonidia are all of the same size, but occasionally specimens 

 have been found in which one »r more dwarf cells occur 

 among them. These are irregular in their distribution and 

 can be distinguished at once from the vegetative cells by 

 their position. Similar dwarf cells were found in both Pan- 

 donna and Eudorina. In matured colonies gonidial cells are 

 frequently found which fail to divide. The gonidia are of a 

 light green colour, a trifle darker than the vegetative-Cells. 

 Their colour in general is similar to that in Eudorina, and is 

 somewhat lighter than that in Volvox and Pandorina, with 

 which they are associated. 



A distinct cell-membrane (cm.) is found about each of the 

 cells. In the living condition and in the material preserved 

 in formalin it forms a highly refractive hyaline layer, about 

 1 /m in thickness, outside of the green contents of the cell. It 

 stains very faintly in hsematoxylin and assumes a deep brown 

 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 (PI. V. 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 



