Volvox in South Africa. 545 



surface of the colony, regarding it as a sphere of diameter equal to the 

 mean diameter of the spheroid, divide by the area of the cell regarded 

 as a regular hexagon ; result, the number of cells in the coenobium. 

 Obviously this method is only very approximately correct ; the 

 diameter of a cell and that of the spheroid are quantities of a different 

 order of magnitude, and the measurement of the former can only be 

 approximate with the means at one's disposal, the probable error in 

 proportion to the size of the diameter being consequently very high. 

 Further, the size of the cell varies in different parts of the coenobium. 



(2) The second method, based on the same considerations, has 

 been reduced to a formula by Janet (1912, p. 28) : the square of the 

 number of cells seen on a great circle multiplied by -367 gives the 

 total number of cells composing the colony. This should give com- 

 paratively more accurate results, but here too the percentage of 

 probable error is high, since, owing to the great number, it is 

 by no means easy to count the cells round the edge of the colony 

 accurately. 



Theoretically, since the colony is held to arise by successive 

 simultaneous bipartitions from a single mother-cell (the gonidium), 

 the resultant number of cells should always be a power of two. 

 Zimmermann (1921, p. 260, note 2), in the case of V. aureus, states : 

 ' The cells of a colony divide constantly simultaneously, and in a 

 healthy culture degenerating cells scarcely occur." And again (1925, 

 p. 397, and note 3) : ' Die Furchung . . . ist eine fortgesetzte 

 synchrone Zweiteilung samtlicher Zellen," adding to this a note 

 that, according to older statements, from the 8-celled stage onwards 

 a part of the cells may no longer divide. 



This statement may be correct as regards V. aureus, where the 

 total number of cells is 4096 at the outside, often as few as 1024. 

 In the case of the globator section, however (Eu-volvox), the number 

 of cells in question is very much higher, and in the two South African 

 species at any rate there are indications that the cells do not all 

 divide simultaneously, that in a division some cells may fail to 

 participate without in any way degenerating, sometimes merely 

 missing a division. Further, in V. capensis the gonidia are clearly 

 differentiated before the daughter colony inverts, and always have 

 at least two pyrenoids. The conclusion seems obvious that they are 

 cells which have not taken part in at least the last, probably 

 several cell-divisions (fig. 1 E). 



If Zimmermann's statement were invariably correct, the possible 

 numbers of cells, including the reproductive cells, would be four only, 



