560 Annals of the South African Museum. 



Comparison of Cell Structure in V. Rousseletii and V. capensis 



with that of V. globator. 



From the foregoing account it will be seen that the structure in 

 the two South African species in question is essentially similar to 

 that of V. globator as described by the workers quoted, though 

 differing from these descriptions in some details. It must, however, 

 be emphasised that, whereas in these two species the structure has 

 been studied in the living cell, V. globator has only been seen by the 

 writer preserved or in section, so that for comparison of cell structure 

 it has been necessary to depend entirely on the published accounts. 

 To study the form of the chloroplast in particular, living material is 

 essential " the best stain for protoplasm is chlorophyll ' it is 

 possible that if V. globator were studied side by side with these two 

 South African species the apparent differences in cell structure would 

 disappear, particularly as regards the shape of the chloroplast and 

 the distribution of the contractile vacuoles. 



In spite of the very marked likeness to the known structure of 

 V. globator, the cell constituents have been described here as fully as 

 possible, since only thus can we hope to arrive at a clear conception 

 of the group as a whole and of the inter-relationships between the 

 species. Lack of such detailed descriptions from living material is 

 responsible for a great deal, if not all, of the confusion in our know- 

 ledge of the whole genus or group of genera. In this connection the 

 remarks of Pascher in his account of the genus Volvox may be again 

 recalled (1927, p. 450). 



It may be as well to emphasise here the very great advantage 

 gained by using the TV-inch oil-immersion objective for examination 

 of the living cell ; details in structure, as, for example, the dorsal 

 extension of the chloroplast, can be seen clearly examined thus which 

 can hardly be distinguished with the usual high-power objective. 



Reproduction. 



Differentiaton of Reproductive Cells. As already stated, at an 

 early stage of the development of the daughter colonies reproductive 

 cells are distinguishable from the somatic cells by their greater size 

 and two or more pyrenoids. The stage at which this is first seen 

 varies, but in both species it is long before birth (Plate XXXIX, B). 



In the cultures of V. Rousseletii from Kietfontein, at birth these 

 cells, if asexual gonidia, are about twice the diameter of the vegetative 

 cells, with a large well-marked nucleus. As regards the sexual cells, 



