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 demonstrate that both prostaglandin D2 and prostaglandin E2 exert hyperalgesia in the spinal cord , but in different ways .
### Output Text:
These results demonstrate that both <span class="chemical">prostaglandin D2</span> and <span class="chemical">prostaglandin E2</span> exert hyperalgesia in the spinal cord , but in different ways . 