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: Composition of gall bladder stones associated with octreotide : response to oral ursodeoxycholic acid .
### Output Text:
Composition of gall bladder stones associated with <span class="chemical">octreotide</span> : response to oral <span class="chemical">ursodeoxycholic acid</span> . 