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: Despite the advent of calcium channel blockers and beta - adrenergic antagonists , ergot preparations continue to play a major role in migraine therapy , so that the danger of St .
### Output Text:
Despite the advent of <span class="chemical">calcium</span> channel blockers and beta - adrenergic antagonists , <span class="chemical">ergot</span> preparations continue to play a major role in migraine therapy , so that the danger of St . 