546 Annals of the South African Museum. 



viz. 8192 (2 13 ), 16,384 (2 14 ), 32,768 (2 15 ), and 65,536 (2 16 ), since no 

 observer yet has computed more than 50,000 cells per colony. Even 

 allowing a large margin of error in counting the cells, these numbers 

 scarcely cover all observed variations. Irregularities in division, 

 e.g. a number of cells at one stage or another undergoing an extra 

 division or failing to participate in a division, would account very 

 simply for considerable variation in the number of constituent cells. 

 As a strain ages, or if external conditions are unfavourable (e.g. 

 in cold weather), not only does the size of the coenobium tend to 

 decrease, but the number of cells per coenobium also decreases, some- 

 times proportionately more than the size. In such cases the cells 

 (or rather, protoplasts) are usually larger, and the colonies as a con- 

 sequence appear different, the network being coarser in texture, i.e. 

 there is a tendency to return to the type of structure found in the 

 very early (second and possibly third) generations of the strain. This 

 has been observed repeatedly in cultures of the Rietfontein V. Rous- 

 seletii and in V. capensis on the Flats. Thus, in both very young and 

 very old strains, the cells are usually fewer and larger than in strains 

 that are at the height of their development. 



Types of Cells composing the Coenobium. 



Two types of cells are found in the coenobium : (1) Vegetative or 

 somatic cells, the " Arbeitzellen " of Klein (1890, p. 47), forming the 

 ' working partnership " which Klein regarded as mainly nutritive. 

 These cells have lost the power of further division. (2) Specialised 

 reproductive cells which are differentiated from the somatic cells 

 at an early stage prior to the inversion of the embryo, either at the 

 last cell-division or earlier. These are of three kinds : (a) Gonidia 

 (the " Parthenogonidia ' : of the earlier writers), which give rise 

 by repeated division to the daughter colonies ; (b) male initial cells, 

 which in the same way give rise to globoids of spermatozoids ; and 

 (c) female initial cells, which develop into oogonia, each of which 

 contains a single oosphere (Plate XXXIX, E). 



The reproductive cells are never found at the anterior pole ; the 

 gonidia are confined to the posterior half, the sexual initial cells to 

 the posterior two-thirds or three-quarters of the coenobium. 



The Somatic Cell. 



The somatic or vegetative cell consists of a ciliated protoplast 

 enclosed within more or less gelatinous walls bounded by limiting 



