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: Vincristine was accidentally given intrathecally to a child with leukaemia , producing sensory and motor dysfunction followed by encephalopathy and death .
### Output Text:
<span class="chemical">Vincristine</span> was accidentally given intrathecally to a child with leukaemia , producing sensory and motor dysfunction followed by encephalopathy and death . 