Volvox and Associated Algae from Kimberley. 485 



bowl-shaped structure is well defined in the 4- and 8-celled stages 

 (fig. 2, E-G), becoming more and more pronounced as development 

 proceeds. Once division has started it proceeds rapidly to completion, 

 so far as could be ascertained following the scheme of division already 

 described by Janet (1923, p. 109 et sec., Plates VII to XVII) for Volvox 

 aureus (Janetosphaera aurea) ; as in that species, the number of cells 

 is comparatively small, despite the much greater size of V. gigas. 

 Usually the number of cells is about 1000 (2 10 =1024), i.e. there are 

 probably at least 10 successive divisions in normal embryos, and not 

 more than 12, since the highest number of cells counted was about 

 3000 per coenobium. 



Inversion. Completion of cell division is followed by inversion of 

 the embryo, resulting in the characteristic football-shaped daughter 

 colony lying between the inner and outer membranes of the parent. 

 In proportion to the number of embryos, the number of inversion 

 figures seen in the material was extraordinarily small. This may be 

 due to the fact that the process of inversion is a rapid one, or in part 

 at any rate to the time of collecting, since the evidence in other species 

 points to an optimum time for inversion about midday (Pocock, seq. 

 p. 579). Inversion figures are commoner in some tubes than in others, 

 though never very abundant. In spite of their comparative rarity, 

 however, sufficient were seen to indicate the method of inversion 

 which, as was to be expected, is a simple one, on the whole following the 

 lines described for F. tertius Meyer by Kuschakewitsch (1922 and!931),* 

 but in some ways more like that seen in sperm bundles of V. capensis. 



* Kuschakewitsch (1931, p. 328) states that the stages in inversion in V. aureus 

 are so like those in V. tertius that a separate description for the former species was 

 not necessary in his short paper. Janet (1923, p. 127, and pi. xviii) agrees with 

 Kuschakewitsch, giving a single description for the process in the two species. 

 Zimmermann (1925, p. 51), however, working with F. aureus, found considerable 

 difference in detail in that species from the description given by Kuschakewitsch 

 for V. tertius. Thanks to the kindness of Mr. Scourfield, samples of both I", aureus 

 and V. tertius from Europe were available for comparison. These contain a few 

 good inversion figures of which those in V. aureus include an early stage and the 

 stages figured by Zimmermann as fig. 2, g and h, while in V. tertius stages corre- 

 sponding to Kuschakewitsch, Taf. 20, B and E, and Janet, pi. xviii, C and D, 

 were seen. Hence it seems probable that the descriptions and figures given by 

 Kuschakewitsch and Janet must be regarded as applying to V. tertius only, while 

 those given by Zimmermann are correct for F. aureus. 



The fundamental difference in the two methods is that in the F. tertius type of 

 inversion actual inversion starts by the lip of the phialopore turning outwards, 

 while in F. aureus inversion begins behind the lip, which remains doubled inwards 

 until a late stage, when it finally straightens out. A comparison of fig. 2, H-N, 

 with fig. 5, B-F, will make this clear, since in V. africanus inversion is of a similar 

 type to that in V. aureus. 



