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: In those subjects who had no migraine attack ( 11 subjects ) a similar time schedule was used .
### Output Text:
In those subjects who had no migraine attack ( 11 subjects ) a similar time schedule was used . 