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: A discussion of the history of ergot includes its original discovery , the epidemics of gangrene that it has caused through the ages and its past and present role in the management of migraine headache .
### Output Text:
A discussion of the history of <span class="chemical">ergot</span> includes its original discovery , the epidemics of gangrene that it has caused through the ages and its past and present role in the management of migraine headache . 