Volvox in South Africa. 579 



Fuller observations are necessary before reaching a final conclusion 

 on this point', not only on the time of inversion itself, but also as 

 regards the time at which nuclear division takes place. There is 

 probably a close relation between the two, since inversion follows 

 closely on completion of division. Shaw speaks of ". . . night . . . 

 a time when so many of the changes in the life-history of Volvox 

 occur." Karsten (1918), working with various algae, found that, in 

 general, nuclear division is initiated by change in light intensity, e.g. 

 from light to dark, and concludes that by day the cells assimilate C0 2 , 

 and at night use up the food stored during the day in cell-division. 



In Volvox as yet it has not been possible to determine whether 

 cell-division is confined to certain periods of the 24 hours. So far 

 as external observations go, the results obtained do not support Shaw's 

 remark so far as concerns the African species. In V. aureus Zimmer- 

 mann (1921, p. 270) certainly found nuclear division proceeding during 

 the day, as he describes various stages of cell-division from material 

 fixed at 3 p.m. 



The results of observations as to the time at which inversion takes 

 place though inconclusive are perhaps worth recording. Although 

 inversion was at various times observed at all hours of the day, 

 there seem to be certain optimum times which, however, depend to 

 some extent on external conditions. Material examined between 

 8 and 10 a.m. usually showed many daughters in the preparatory 

 stages. From 10 a.m. to about noon many inverting daughters 

 were seen ; thence onwards the number usually decreased markedly, 

 increasing again somewhat towards the late afternoon or early 

 evening, after which it again falls. Inverting daughters were more 

 numerous in warm weather, but this was to be expected since a rise 

 in temperature evidently speeds up development in general. 



Fixation. To obtain good inversion figures the best method was 

 found to be killing with dilute osmic acid, then washing in several 

 changes of water, and fixing in dilute chrome-acetic, as used in 

 Flemming's Fluid No. 2, overnight. Good results were also obtained 

 by using Powers' iodine method. 



The Discovery of Inversion. 



It is hardly credible that in so well known an organism as Volvox, 

 which has been investigated again and again during the last two 

 centuries, a phenomenon so striking as that of inversion of the daugh- 

 ter colonies should have been completely overlooked. Yet such is 



