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characteristic sheen on the pustules. One of us favours Owen’s 
description, and the other Carruthers’, but we both agree that 
the colour is best defined by Ridgway’s “ Dusky Purplish Gray.”’(°) 
The colour of the depression surrounding the pustule is similar 
to that of the pustule itself, and when many pustules coalesce 
a brownish purple patch is formed. No difficulty need be experi- 
enced, therefore, in distinguishing the pustules from the closed 
sori of Corky Scab of the same size. 
Internal Structure. The internal structure of the pustule is 
also very distinctive in its characters, and we do not feel that 
the figures given by other investigators are sufficiently detailed 
in this respect. Shapovalov’s figure(!°), in fact, could be more 
easily taken for Corky Scab than Skin spot. We have sectioned 
over a hundred pustules in all stages taken from Ally, King 
Edward and Kerr’s Pink potatoes and find the variation in 
structure surprisingly slight. Fig. 6 gives a composite drawing 
in which we have introduced every feature which we have met 
with in the disease, but, as already stated, the deviations from 
type are so slight that the figure may be taken as representing 
a typical Skin spot. A sbort description of the characters shown 
is given below :— 
1. A mature pustule when cut across is found to extend to 
a depth of from 1 to 2mm. It is black at the surface, 
shading to brown towards the base. 
2. The most noticeable feature in microscopic examination 
is the presence of numerous cells with dark brown and 
much thickened walls. These cells are more or less 
empty, and their contents, if any, are brown and dis- 
organised. Many of them are hypertrophied but the 
enlarged cells never show hypertrophy at right angles to 
the epidermis in the way that Kunkel shows to be so 
characteristic of the “giant” cells in a young Corky 
Seab pustule. In mature pustules some of these 
diseased cells may be broken and crushed, but the 
areas of complete disorganisation so formed are not 
confined to the apex of the pustule as suggested by 
Shapovalov’s“® diagram. Miss Owen describes these 
cells as ‘‘ cuticularised,” and this term may serve to 
distinguish them, but, on staining sections in Sudan Ii, 
only those cell walls which have just started to thicken 
stain red. The thicker walls do not take up the stain. 
3. Interspersed with the “ cuticularised ” cells are groups of 
clear cells with thin hyaline walls. These cells are more 
or less empty, but a nucleus, starch grains and proto- 
plasmic contents in varying stages of degeneration may 
often be seen. 
4, A well-marked cup-shaped layer of cork often six or more 
some pustules this layer is not so well developed as in 
