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: In animals pretreated with nicotine for 14 days , the nicotine - induced locomotor response remained to be potentiated .
### Output Text:
In animals pretreated with <span class="chemical">nicotine</span> for 14 days , the <span class="chemical">nicotine</span> - induced locomotor response remained to be potentiated . 