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: These responses were blocked by systemic pre - administration of hexamethonium chloride ( 20 mg / kg ) .
### Output Text:
These responses were blocked by systemic pre - administration of <span class="chemical">hexamethonium chloride</span> ( 20 mg / kg ) . 