528 Annals of the South African Museum. 



usually the addition of a drop of 1 per cent, osmic acid to the water 

 containing the Volvox. By the use of a Davis shutter for attachment 

 to the microscope objective, kindly lent by Mr. Steer, additional 

 focal depth was obtained ; this was most useful in photographing the 

 colonies in the round. Cell detail was photographed under higher 

 powers in many cases it was found possible to observe and photo- 

 graph successfully, while actually living, single cells or groups of cells, 

 using a 1/12" oil-immersion lens. In this way a surprising amount of 

 detail as nearly free from distortion as possible could be obtained. 

 Further, by the use of panchromatic plates, in some cases, e.g. in the 

 study of oospores, details of structure almost or completely imper- 

 ceptible by the eye are recorded by the camera. 



In the case of inverting daughter colonies, the pressure of the 

 cover-slip supported on pieces of cover-slip glass keeps the parent 

 coenobium in position without interfering with the process of inver- 

 sion, so that it could be followed photographically by a series of 

 fairly high-power photographs taken at intervals of a few minutes. 

 The male bundles are, of course, very much smaller, and in the 

 study of their development the oil-immersion objective (Bausch and 

 Lomb, 1-9 mm.) was used with a Leitz 3 optical. In the process of 

 photographing with high powers there was often considerable diffi- 

 culty owing partly to the fact that the slightest change in pressure 

 on the cover-slip may cause the object to slide slowly out of the 

 field, but more particularly because here, as in the developing daughter 

 colony, the whole structure and the individual cells composing it are 

 slowly but continuously moving and altering their shape. Thus the 

 range of possible exposure is very limited. In the case of inverting 

 sperm bundles when the fastest available panchromatic plates were 

 used with a powerful projector illuminant, the minimum exposure 

 for good results was 8-10 seconds, and even this usually gave slight 

 blurring in some parts owing to the movement of the cells (cf. Plates 

 XLIII and XLIV). This, of course, applies much more to the cilia, 

 of which in living cells only the basal, non-moving parts appear 

 clearly in the photograph, the upper parts showing only as a blurred 

 outer zone parallel to the surface of the colony (see Plate XIX, A).* 

 In addition to the photographs, careful drawings were made 

 throughout. 



Records were kept as to the state of the Volvox in Mr. Steer's 



* For reasons of economy, photographs and diagrams already reproduced in 

 the former paper on Volvox in Africa (Rich and Pocock, 1933) are not repeated 

 here, although referred to in the text. 



