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: Thirty - three children after dexamethasone treatment were matched to 33 children without dexamethasone treatment .
### Output Text:
Thirty - three children after <span class="chemical">dexamethasone</span> treatment were matched to 33 children without <span class="chemical">dexamethasone</span> treatment . 