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: Pre - treatment with ketamine produced cross - tolerance to morphine , whereas pretreatment with morphine did not induce cross - tolerance to ketamine but rather augmented the cataleptic response ; this augmentation was attributed to residual morphine in the brain .
### Output Text:
Pre - treatment with <span class="chemical">ketamine</span> produced cross - tolerance to <span class="chemical">morphine</span> , whereas pretreatment with <span class="chemical">morphine</span> did not induce cross - tolerance to <span class="chemical">ketamine</span> but rather augmented the cataleptic response ; this augmentation was attributed to residual <span class="chemical">morphine</span> in the brain . 