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: Fatal myeloencephalopathy due to accidental intrathecal vincristin administration : a report of two cases .
### Output Text:
Fatal myeloencephalopathy due to accidental intrathecal <span class="chemical">vincristin</span> administration : a report of two cases . 