Volvox in South Africa. 577 



p. 401) states that it remains recognisable as a clear " cell-free " 

 spot, apparently in V. globator as well as in V. aureus, but he is not 

 quite clear on this point. In these African species certainly no such 

 spot can be traced. 



The cilia are still very short on completion of inversion, and con- 

 tinue to elongate for some time. 



Results of Inversion. 



Inversion is now complete, the inner pole of the developing daughter 

 has become the anterior pole, while the phialoporic pole is now the 

 posterior pole of the rotating daughter. Further, the orientation of 

 each cell has been inverted, the polar axis has been turned through 

 an angle of 180 so that the pole innermost in cell-division is now 

 outwards, while the chloroplast-containing end is on the inner side 

 of the young colony. The elongation of the polar axis of the young 

 colony is sometimes apparent immediately after inversion, sometimes 

 does not appear till later. Occasionally the daughters remain practi- 

 cally spherical till after birth. 



After inversion the daughter is actually slightly smaller in size 

 than on completion of cell-division, since the long axis of each cell 

 is now the radial one. The cells are closely packed together and have 

 entirely lost their sperm-like appearance ; in an optical section of a 

 young inverted colony the cells appear oblong in form, both outer and 

 inner ends broad and slightly rounded (fig. 2, A, and fig. 4, C ; Plate 

 XXXIX, A). The outer end is colourless, containing the fairly large 

 nucleus and bearing the two cilia, while the inner two-thirds is filled 

 by the deep-green chloroplast with the pyrenoid. 



The above details have been obtained primarily by continuous 

 observation of living material. Daughters have been watched during 

 periods of several hours, from the very first sign of preparation for 

 inversion that could be detected until inversion was complete, and 

 it is only such continuous observation that allows the sequence of 

 events to be followed. This has been supplemented by examination 

 of sections of material specially fixed with a view to obtaining good 

 inversion figures (hence killed with dilute osmic acid or iodine solution). 

 Such preparations confirmed the observations already made in living 

 material of the change in shape of the cells, showing the very sharp 

 pointing of the ends (fig. 4, B, and Plate XLI, H). Further, in iodine- 

 fixed material the rudimentary cilia can be seen at a very early 

 stage. Apparently the elongating and pointing of the cells is directly 



