Volvox in South Africa. 553 



division of a pre-existing pyrenoid or de novo. Microtomed sections 

 of iodine-fixed material yield some evidence to show that the pyrenoids 

 divide, and examination of young developing daughters in life tends 

 to support this theory of the origin of the pyrenoids from pre-existing 

 ones, but further investigation is necessary to show if this is always 

 the case. Zimmermann (1921, p. 264), like Overton (1889, p. 147), 

 at first regarded the formation of pyrenoids de novo as probable, 

 but later (1925, p. 397, note 2), having given more attention 

 to living material, saw reason to doubt the validity of his earlier 

 view. 



The form of the pyrenoid and the starch secreted by it, as seen here 

 in living or freshly killed material, agree very closely with the 

 observations made by C. Lander (1929, p. 433) in preserved and 

 sectioned material of V. globator. 



In older vegetative cells as well as in reproductive cells, besides 

 the starch-sheath surrounding the pyrenoid, there are often minute 

 scattered grains of starch. Older cells often contain in addition 

 one or more fairly large, highly refractive globules, probably of 

 volutin, which in unstained material may give to the cell the appear- 

 ance of possessing more than one pyrenoid (fig. 1, C). 



(iv) Cilia. Every Volvox cell when young has two cilia ; the 

 reproductive cells lose theirs at an early stage of development, but 

 the somatic cells retain theirs throughout their life. 



The cilia are long, about treble the length of the protoplast (23 p. 

 approximately), delicate, and colourless; they can, however, usually 

 be fairly easily seen in living colonies even at comparatively low 

 powers of magnification particularly beautifully in young daughter 

 colonies rotating within the vesicle before birth. The rapidly moving 

 cilia en masse appear like a greyish, undulating sheath surrounding 

 the young deep-green colony (cf. Plate XLVIII, F). 



The cilia are inserted separately at the apex of the cell (fig. 1, A-D), 

 one on each side of the strap-shaped extension of the chloroplast, 

 and are always definitely orientated the line joining the bases of a 

 pair of cilia is at right angles to the antero-posterior plane of the cell. 

 Thus the members of a pair of cilia are equidistant from the pole of 

 the colony. Consequently, if the cells of a colony have suffered no 

 distortion, and the colony is lying on the side, the two cilia in most of 

 the cells are in the same line of vision and appear to be inserted at 

 the same spot, except near the poles. If, however, the colony is 

 lying with its polar axis at right angles to the slide, optical section 

 gives an equatorial view, the majority of the cells are seen in either 

 VOL. xvi, PART 3. 36 



