Plate 24 



A. A pair of coalescing " keratin pearls", cysts lined with a squamous, 

 stratified, keratinizing epithelium in a tongue of invasive epidermal tissue 

 on the skin of a mouse treated with benzpyrene. Light micrograph, 

 stain: haemotoxylin-eosin. Preparation made by Dr. I. Hieger. 



B. Light micrograph of cross-sections of wool fibres to show the dis- 

 tribution of o- and p-type cells. The fibre has been oxidized with pera- 

 cetic acid and stained under acid conditions with methylene blue (Pearse's 

 technique (1951)). The enhanced basiphilia (the dark stain) indicates the 

 regions of higher sulphur content (para-type). Note that although the 

 ortho-para distribution is predominantly bilateral, para-ce\\s may be 

 found among the ortho. 



