486 Annals of the South African Museum. 



A colony with undivided gonidia and developing embryos, including 

 several in various stages of inversion, is shown in fig. B, Plate XXVI. 

 The earliest stage seen (fig. 2, H) shows a widening 4-sided phialopore 

 with 4-lobed lip, depressed and curled inwards, while the whole 

 embryo shows some denting and distortion. In the next stage (fig. 2, 

 J, K) the denting disappears, a slight constriction forms, and the lip 

 begins to straighten out and rise. The phialopore continues to widen 

 and the lobes turn outwards, becoming very distinct in the process ; 

 the cells at the edge are as a consequence in a state of tension, slightly 

 separated from one another, and at this stage protoplasmic con- 

 nections between the cells could be clearly seen (fig. 2, L, and Plate 

 XXVII, A). These connections continue to show distinctly in the 

 lobes as they fold back (fig. 2, M, and Plate XXVII, B, C). Character- 

 istic " hat " stages follow (fig. 2, N, Plate XXVI, F, and Plate XXVII, 

 D), and finally the edges of the phialoporic lobes meet and inversion 

 is complete (Plate XXVII, E). At first the phialoporic side is flatter 

 than the anterior pole, the ends of the elongated cells are rounded and 

 separated from one another, and the very short developing cilia are 

 visible (fig. 2, 0, and Plate XXVII, F). The cilia lengthen rapidly, 

 the cells alter in shape, becoming shorter, wider, and more compact, 

 and the common envelope is formed (Plate XXVII, G) ; enlargement 

 follows, and soon the gonidia are distinguishable from the somatic 

 cells (Plate XXX, D). 



The whole process of inversion is very like that observed in the germ 

 colony formed on germination of the oospore of Volvox Rousseletii (cf. 

 succeeding paper, p. 622) ; there, on completion of cell division the 

 hollow globoid lies freely in a fairly wide vesicle much as does the 

 young embryo of V. gigas, which, owing to the absence of intercellular 

 protoplasmic strands, has no connection with the neighbouring 

 somatic cells. The surrounding vesicle in the latter shows clearly 

 in several of the microphotographs, e.g. Plate XXVII, C, D. This 

 type of inversion would appear to be the primitive one characteristic 

 of the simpler multicellular members of the Volvocales, in which the 

 cell-number is several hundreds, but in general not more than 1000 to 

 2000. 



A very interesting point emerged from the study of the inversion 

 figures in this species. In his description of V. tertius, Meyer (1896, 

 p. 200) states that although in the adult coenobium there are no proto- 

 plasmic connections between neighbouring cells, yet in living unborn 

 embryo colonies such connections are readily visible. In both V. gigas 

 and V. africanus protoplasmic connections can occasionally be seen 



