Dr. C. A. Koibiil on Platydorina. 549 



marginal ones, often exhibit a slight flattening or even a 

 depression at the outer pole. In the full-grown colony the 

 cells have an equatorial diameter of 15-20 fi and a polar one 

 of 15-18 /j,. The cells of young colonies still within the 

 maternal matrix do not exceed 4-6 fi in diameter. I do not 

 find that the cells of the 16 cell colonies are appreciably 

 larger than those of the 32-cell. 



The protoplasm is small in amount, consisting of a very 

 thin pellicle (fig. 4, jt>.) on the surface of the cell on the out- 

 side of the chromatophore, and an axially-placed knob- 

 shaped mass (pr.) located somewhat nearer the outer pole 

 than the inner one. Near the centre of this mass lies the 

 spherical nucleus (fig. 4, n.), containing a single spherical 

 nucleolus (net.). Within this protoplasmic mass lie the two 

 contractile vacuoles (c.v.) and the stigma («<.), while from 

 the outer end of the cell arise the two flagella (/.). 



There is but a single cup-shaped chromatophore (fig. 4, 

 chr.), which is inclosed within the pellicle above noted, and 

 itself contains the knob-shaped protoplasmic mass. It is of a 

 brilliant chlorophyll-green colour, and contains numerous 

 small granules of irregular and somewhat angular outline. 

 Towards the inner end of the cell, imbedded in the thickest 

 part of the chromatophore, there is a single spherical py- 

 renoid, having a diameter of 4-6 /a. 



The stigma or eye-spot, seen from above, is circular in 

 outline, but in lateral view has the form shown in tig. 4, s. 

 The slightly convex outer surface appears to project some- 

 what beyond the rounded contour of the cell. The colour i3 

 usually a bright reddish brown, often brightest in the anterior 

 and marginal cells and rarely entirely faded in the posterior 

 ones. The stigma is a homogeneous body, showing no trace 

 of structure beyond the well-defined contour-line, which is 

 best seen in fading and moribund cells. It is normally 

 present in all cells of the colony, and may readily be demon- 

 strated by full illumination. The position of the stigmata in 

 the cells is somewhat unusual, and is significant of the pro- 

 nounced polarity of the organism. The customary position 

 in other genera is adjacent to the bases of the flagella. In 

 Platydorina, however, the location of the stigma is not con- 

 stant with respect to the flagella, but seems rather to bear a 

 definite relation to the form of the colony, since it lies towards 



"inner" are used to designate respectively the ends which bear the 

 stigma and the pyrenoid. 



