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: On the mechanisms of the development of tolerance to the muscular rigidity produced by morphine in rats .
### Output Text:
On the mechanisms of the development of tolerance to the muscular rigidity produced by <span class="chemical">morphine</span> in rats . 