Volvox in South Africa. 547 



membranes, neighbouring protoplasts being connected with one 

 another by strands of protoplasm. As the colony grows, the space 

 enclosed within the cell walls continually increases in size, whereas 

 the protoplasts, which at first grow too, soon cease to enlarge. As 

 a consequence the protoplasts in adult colonies are some distance 

 from one another, separated by clear spaces which are not coloured 

 by the stains normally used, and which are traversed by the proto- 

 plasmic strands connecting adjacent protoplasts. 



Among the somatic cells themselves there is some differentiation 

 intimately connected with light perception. The presence or absence 

 of a pigmented eyespot, its size and position are dependent on the 

 position of the cell in the coenobium, and with this variation in the 

 eyespot go the differences found in the shape of the cell. In the 

 anterior half of the colony the cells show distinct dorsi-ventrality 

 (fig. 1, A, B) ; the colourless ventral side, i.e. that part of the cell 

 which faces towards the anterior pole, is often somewhat flattened 

 and usually contains two small contractile vacuoles, while the eye- 

 spot is situated on the green dorsal side, its position relative to the 

 apex of the cell varying slightly according to the position of the 

 latter in the coenobium (Plates XIX, A ; XXXVIII, E). In the 

 posterior half of the coenobium the dorsi-ventral aspect is less pro- 

 nounced, becoming progressively less so in respect to cell-form as the 

 posterior pole is approached. Even in this region, however, it is 

 still possible to trace a definite orientation of the cell in the coenobium 

 by studying the insertion of the cilia (fig. 1, C). 



1. Size of Cell. The diameter of the cell from membrane to 

 membrane varies according to its position in the colony and the 

 age of the latter. In a very young colony the cells are small and 

 the protoplasts very close to one another, almost touching (Plate 

 XXXVIII, F) ; as the colony develops the cells enlarge, the proto- 

 plasts becoming further apart, particularly in the anterior region. 

 In still older colonies the enlargement of the cells continues, but the 

 protoplasts begin to get smaller, particularly in the posterior half of 

 the coenobium (Plate XIX, B-E). 



The variation in the size of the cell and the protoplast in F. 

 Rousseletii, according to the position in the colony, is indicated in 

 the following table of measurements : 



[TABLE 



