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: Reversal of scopolamine - induced amnesia of passive avoidance by pre - and post - training naloxone .
### Output Text:
Reversal of <span class="chemical">scopolamine</span> - induced amnesia of passive avoidance by pre - and post - training <span class="chemical">naloxone</span> . 