578 Annals of the South African Museum. 



connected with cilia formation. As each part escapes through the 

 widening pore the pointing of the cells disappears again, and the cells 

 assume the palisade-like form described above, and already beginning 

 to be apparent in the section of the " Flask " stage (Plate XLI, I). 



At this stage there is no common envelope. The limiting mem- 

 branes or cell walls are laid down subsequent to inversion. Traces 

 of secretion of mucilage into the hollow of the inverted daughter 

 appear early ; in sections appropriately stained the dilute mucilag- 

 inous content of the daughter colonies, though much less deeply 

 stained than that of the parent, is clearly contrasted with the clear, 

 unstained cavity of the vesicle surrounding the colony. 



Kuschakewitsch (1931, p. 330) suggested that the secretion of 

 intercellular jelly which "according to Overton" (1889) intervened 

 between completion of cell-division and appearance of the cilia, 

 might be in causal connection with the process of inversion (cf. also, 

 Janet, 1923, p. 129). Again, Zimmermann (1925, p. 400) states 

 that the process of " Exkurvation" is manifestly caused by the 

 swelling of the intercellular substance between the apical cell ends. 

 These explanations seem very inadequate. The fact that the 

 emission of the cilia coincides with the beginning of the actual 

 process of inversion and is not subsequent to inversion, however, 

 offers a much more probable explanation of the mechanism of in- 

 version, while the changes that take place in the shape of the cells 

 in the earlier stages might account for many of the phenomena 

 observed. It is possible that the formation of mucilaginous substance 

 between the cells assists in the process. The existence of protoplasmic 

 continuity between the cells enables the whole mass to act as a single 

 entity (see Pocock, loc. cit., p. 486). 



Time of Inversion. 



Both V. Rousseletii and V. capensis were examined at all hours 

 of the day and most of the hours of the night, except between 5 and 

 8 a.m. Inverting daughters were observed at all hours of both day 

 and night. At first it was thought that inversion took place only 

 during the day, and that by bringing coenobia that had daughters in 

 the stage succeeding completion of division from the dark into the 

 bright light of the microscope ilhiminant, these were stimulated to 

 invert. But material which had been standing in the dark was 

 examined at 8.45 p.m. and showed daughters already half inverted, 

 hence in this case, at any rate, inversion had started in the dark. 



