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: Morphine did not , however , alter the hyperalgesia induced by 8 - bromo cyclic adenosine monophosphate .
### Output Text:
<span class="chemical">Morphine</span> did not , however , alter the hyperalgesia induced by <span class="chemical">8 - bromo cyclic adenosine monophosphate</span> . 