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: Rats , tolerant to ketamine - dominant combinations , were cross - tolerant to both drugs , while those tolerant to morphine - dominant combinations were cross - tolerant to morphine but showed either no cross - tolerance or an augmented response to ketamine .
### Output Text:
Rats , tolerant to <span class="chemical">ketamine</span> - dominant combinations , were cross - tolerant to both drugs , while those tolerant to <span class="chemical">morphine</span> - dominant combinations were cross - tolerant to <span class="chemical">morphine</span> but showed either no cross - tolerance or an augmented response to <span class="chemical">ketamine</span> . 