INTRODUCTION 251 



During 1930-1, though able to examine all the scrapings myself, I was almost entirely 

 indebted to Dr F. D. Ommanney and Mr A. H. Laurie for the notes as to the presence 

 or absence and disposition of the film made on the plan. During that season 420 whales 

 were examined. 



Apart from the positive evidence furnished by these collections mention should be 

 made of the valuable observation of Dr Ommanney and Mr Laurie that none of the whales 

 examined at Durban during the southern winter of 1930 showed any trace of diatom film. 



The extent and disposition of the film, if present, were noted soon after the whale was 

 drawn out of the water, and a scraping taken from it with a knife and preserved in a 

 small labelled tube with formalin sea-water for microscopic examination. Care was 

 exercised that these scrapings were taken from portions of the whale's body that had 

 not been in contact with foreign objects. Occasionally fresh scrapings for immediate 

 observation were also taken. 



The examination of the preserved samples was usually completed on the day of their 

 collection. A temporary wet mount was made by pipetting three large drops of the 

 sample on to a clean slide and covering them with a large rectangular cover-glass. This 

 mount was examined for the size and phase of life of the individuals and for the presence 

 of species other than the peculiar skin-film form, for a definite period of time. At first 

 twenty minutes were spent on each sample ; later, when a higher degree of proficiency 

 had been attained, this was reduced to twelve minutes. A 4- in. objective was used. 

 This was found better than the more usual ^ in., the slightly higher magnification being 

 of great advantage in the examination of such minute organisms, while at the same time 

 the use of more cumbersome oil-immersion objectives was avoided. A mechanical stage 

 was of course essential. 



Samples of unusual interest were retained for further study. Improved methods for 

 the making of permanent mounts of diatoms such as have been devised recently by 

 Ghazzawi (1933, pp. 168-9) were not known at the time, but good mounts were ob- 

 tained by taking the diatoms up through alcohols into xyol in a hand centrifuge, and 

 mounting in a medium composed of a-bromo-naphthalene and Canada balsam, de- 

 scribed and recommended for use with diatoms by Edwin E. Jelly in Nature (February 

 22, 1930). Ghazzawi's methods would be highly suitable for making permanent mounts 

 of skin-film diatoms, and may indeed be confidently recommended for use with any 

 strongly silicified forms. 



The drawings on Plate XI have all been made to a uniform scale of i : 1000. 

 Except for the outlines, the camera lucida was not employed, as the curvature of the 

 valves of the various species of Cocconeis is such that it is impossible to focus the whole 

 surface under high magnification. Moreover, the valves are curved in two planes, like 

 cylindrical lenses. Microphotographs are unsatisfactory for the same reason, but were 

 of great assistance in determining the sizes of the frustules. 



For demonstrating the actual penetration of the epidermis by diatoms, which occasion- 

 ally takes place in the vicinity of old scar tissue, sections lO/x and even less in thickness 

 were cut by Mr A. Saunders, the laboratory assistant at Grytviken, and stained with 



