Volvox in South Africa. 587 



whole show well in the series chosen, and the next three photographs 

 (Plate XLIV, A-C), though of another globoid, serve sufficiently 

 well to complete the pictorial record of inversion, while the third 

 series (Plate XLIV, D-F) illustrates rather nicely individual varia- 

 tion in form, and shows the " Hat " stage viewed from a different 

 angle. 



V. Rousseletii. 1. Period of Enlargement and Shifting of 



Polar Axis. 



The young male initial cell is similar in form and structure to the 

 gonidium more or less broadly pear-shaped, with clear anterior 

 end bearing two cilia and containing a large nucleus with well-defined 

 nucleolus (Plate XLII, A, B, D), there is a massive cup-shaped 

 chloroplast with several pyrenoids and a varying number of contractile 

 vacuoles. Owing to their large number and presence still undivided 

 in quite large mature colonies, it is much easier to observe the early 

 stages than in the case of the gonidium. Unlike the gonidium, the 

 male initial cell does not broaden out before division commences. 

 At first it stands in rank with the neighbouring somatic cells (Plate 

 XLII, A, and fig. 5, A), its apex on a level with theirs, distinguished 

 from them only by its greater size and dense cell contents ; even 

 at this stage the inner wall begins to protrude into the hollow of the 

 parent, thus commencing the enlarging process whereby the sur- 

 rounding vesicle is formed. As the cell enlarges the pyrenoids 

 multiply, and the cell begins to drop inward away from the upper 

 surface, the vesicle enlarging inwards correspondingly (fig. 5, B). 

 The cilia disappear (Plate XLII, C, E), the apex occasionally getting 

 somewhat drawn out, and for a time the bases of the cilia may be 

 traced between the cell apex and the outer membrane (fig. 5, C). The 

 fully grown cell has a diameter of about 15 to 20 /JL (cf. Overton, 1889, 

 p. 213, in the case of V. globator, " Gerade vor der Theilung besitzen 

 die Antheridium-Elternzellen meist einen Durchmesser von 15 /j, "). 



Up to now the polar axis of the mother-cell has been radial to the 

 parent colony, hence at right angles to its surface, but at this stage 

 there usually takes place a re-orientation of the cell constituents 

 without any disturbance of the outer cytoplasmic layer, the ectoplasm, 

 which remains as it is, connected with the neighbouring cells by the 

 protoplasmic connecting strands. The whole chloroplast, with all the 

 cell constituents enclosed in it, swings round until the anterior pole, 

 the clear end of the cell containing the nucleus, points sideways or 

 inwards (fig. 5, D to F; Plate XLII, F). Thus the physiological 



