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: Separate times for administering vincristine and intrathecal therapy is recommended .
### Output Text:
Separate times for administering <span class="chemical">vincristine</span> and intrathecal therapy is recommended . 