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: These results suggest that the facilitation of memory retrieval by pre - test morphine might be the direct action of morphine rather than a state dependent effect .
### Output Text:
These results suggest that the facilitation of memory retrieval by pre - test <span class="chemical">morphine</span> might be the direct action of <span class="chemical">morphine</span> rather than a state dependent effect . 