574 Annals of the South African Museum. 



The straightening of the lip, almost closing the ostiole, marks the 

 end of the preparatory stage, and is usually slightly subsequent to 

 the final ironing out of the dents and is completed some two hours 

 or more after the first signs of change are seen (fig. 3, D ; Plate 

 XL, F). In Plate XL, B, a colony of V. capensis with 8 daughters 

 all in the preparatory denting stage is shown evidently the dents 

 were already beginning to disappear, and during the time that elapsed 

 between the taking of this photograph and the one shown on Plate 

 XLI, A, of the central upper daughter (lower, as the photograph is 

 mounted), this daughter at any rate had smoothed out, although the 

 phialopore lip is still depressed. The camera was focussed on to the 

 lip, which is obviously at the same level as the region near the outside 

 of the colony, i.e. just above the equator. These colonies already 

 showed well-developed reproductive cells, probably gonidia, some of 

 which are just discernible in the photograph. The parent had split 

 under its own weight in the first photograph, but in the latter it was, 

 of course, under a cover-slip. 



B. Inversion. (4) ' Hour-glass " Stage. Without perceptible 

 break the further changes which produce the actual turning inside 

 out of the colony follow. These changes first become apparent 

 in the equatorial region of the young colony ; a slight constriction 

 appears in this region, or it may be shifted slightly towards either 

 pole (fig. 3, E ; Plate XL, G). Externally this constriction appears 

 slightly darker than the rest of the colony, which has now a shape 

 rather like an hour-glass. In optical section the cells in the region of 

 the constriction are seen to be narrower and more elongated, the inner 

 apex clear, the outer part dense, dark green, and rounded. Micro- 

 tomed sections reveal the fact that at this stage the cilia begin to 

 form, not subsequent to inversion, as Zimmermann (1925, p. 401) 

 and Janet (1923, p. 129) describe in V. globator ; the inner apex of 

 the cell becomes long drawn-out and pointed in the process, and it 

 appears probable that cilia formation and the subsequent activity 

 of the rudimentary cilia play an important part in the process of 

 inversion. In living material the cilia were not observed until a 

 later stage, and possibly they are too small to be seen except in micro- 

 tomed sections. 



(5) Starting from the equatorial constriction these changes gradu- 

 ally spread over the whole posterior portion, which consequently 

 now appears much darker and considerably smaller than the anterior 

 half (fig. 3, F ; Plate XL, H). The posterior cells are narrow, elon- 

 gated, and pointed at both ends, the pointed outer ends often being 



