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: Facilitation of memory retrieval by pre - test morphine and its state dependency in the step - through type passive avoidance learning test in mice .
### Output Text:
Facilitation of memory retrieval by pre - test <span class="chemical">morphine</span> and its state dependency in the step - through type passive avoidance learning test in mice . 