In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Peroxisomes in hepatomas and hyperplastic preneoplastic liver lesions induced in mice by 500 ppm alpha - benzene hexachloride were examined histochemically and electron microscopically .
### Output Text:
Peroxisomes in hepatomas and hyperplastic preneoplastic liver lesions induced in mice by 500 ppm <span class="chemical">alpha - benzene hexachloride</span> were examined histochemically and electron microscopically . 