Volvox in South Africa. 557 



strand near this junction node, but this is usually in older colonies. 

 As the growth of the coenobium continues, the connections become 

 progressively more attenuated and lose their green colour. 



In life, in normal conditions, the protoplasmic connections are 

 usually sufficiently obvious, but in fixed material it is often a very 

 different matter (cf. Plate XXXVIII, G). If the fixative used is 

 alcohol, the connections often disappear more or less completely. 

 Formalin is better if the right strength (about 3 per cent.) is used, 

 but even with formalin they are sometimes lost. From this point of 

 view as well as others the best fixative for use in the field is undoubt- 

 edly an aqueous solution of iodine in potassium iodide (see Chamber- 

 lain, 1924, p. 178), allowed to act for 24 hours or even longer before 

 washing, after which the material is transferred to 10 per cent, gly- 

 cerine. In material fixed by this method the protoplasmic strands 

 are beautifully clear and the cells undistorted (Plate XVII, B). 



Even in living material, however, the protoplasmic strands may 

 sometimes be withdrawn completely, either from a part or the whole 

 of a colony. Colonies have been seen in which the strands had dis- 

 appeared from the anterior quadrant, where the cells were rounded 

 in outline, while elsewhere in the colony the protoplasmic strands 

 were quite normal. In material brought indoors, particularly in 

 warm weather, the connections sometimes disappear. Probably their 

 absence is always a pathological condition. 



Usually the strands connect two cells directly, but not infre- 

 quently, particularly in older colonies, the strands show considerable 

 anastomosing. A strand from one protoplast branches, the branches 

 passing to different protoplasts or different points of the same neigh- 

 bouring protoplast (Plate XIX, E), or neighbouring strands are con- 

 nected with one another by more delicate cross strands. 



A comparative study of the process of inversion of the daughter 

 colonies in the various South African species of Volvox throws 

 interesting light on the probable history of the evolution of the 

 protoplasmic connections in Volvox. The subject has already been 

 discussed briefly elsewhere (Pocock, p. 486), and the data are still 

 incomplete, since as yet it has not been possible to procure living 

 material of the other species in question (V. gigas, V. africanus, and 

 V. aureus). The assumption made there, that the various forms 

 constitute a progressive series culminating in those species which 

 in the adult have well-developed connections between adjacent cells, 

 is based not only on general principles but also largely on the 

 degree of complexity in the development of the daughter colonies, 



