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: At the time of administration of morphine , histological modifications of the bladder wall , such as chorionic and muscle layer edema , were observed .
### Output Text:
At the time of administration of <span class="chemical">morphine</span> , histological modifications of the bladder wall , such as chorionic and muscle layer edema , were observed . 