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: After one week , slices of cortex , outer and inner medulla from one kidney were incubated in buffer and prostaglandin synthesis was determined by radioimmunoassay .
### Output Text:
After one week , slices of cortex , outer and inner medulla from one kidney were incubated in buffer and <span class="chemical">prostaglandin</span> synthesis was determined by radioimmunoassay . 