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: Catalepsy induced by combinations of ketamine and morphine : potentiation , antagonism , tolerance and cross - tolerance in the rat .
### Output Text:
Catalepsy induced by combinations of <span class="chemical">ketamine</span> and <span class="chemical">morphine</span> : potentiation , antagonism , tolerance and cross - tolerance in the rat . 