280 Illinois State Laboratory of Natural History. 



carmine, and is by tins 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 (/•.). In the younger cells 

 the nucleus is much smaller (4-5 /<), 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 

 grayish protoplasmic mass at the center of the cell. The 

 protoplasm is continued from this central region peripherally, 

 in the axis of the cell as a slender column (p. <■.), to the 

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

 two flagella. A protoplasmic mantle enclosing the chromat- 

 ophore 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 color, though in 

 some of the posterior cells the color is often very faint, giving 

 the stigma the appearance of a slightly tinged oil-globule. 

 It is of an elongated hemispherical shape when seen from the 

 side, and has a circular outline when seen from above. Its 

 upper end often projects slightly so as to elevate the cell 

 membrane. The application of killing agents and alcohol 

 soon removes its color, and even in formalin this fades out in 

 the course of a few days, leaving merely a colorless, highly 

 refractive structure. The larger stigmata have a diameter of 

 2.5 /< and a depth of 2.8 //, and are to be found in the cells 

 in the anterior part of the colony, especially in the four vege- 

 tative cells of the anterior polar circle. Posteriorly the stig- 

 mata are less prominent, and are often not to be found at all 

 as brightly colored spots but merely as pale globules whose 

 position alone affords a clue to their real character. 



This specialization of the stigmata in the anterior end of 

 the colony occurs also in Eudorina, Pandorina, and Volvox, 

 and Shaw ('9-i) states that in V. californica the stigmata, 

 which are present in the posterior part of the young colonies 

 (in gonidial cells), become less conspicuous and disappear as 

 the colony enlarges and the differentiation of the cells pro- 



