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: Saline - pretreated controls given a test dose of morphine ( 20 mg / kg i . p . ) showed a pronounced rigidity recorded as tonic activity in the electromyogram .
### Output Text:
Saline - pretreated controls given a test dose of <span class="chemical">morphine</span> ( 20 mg / kg i . p . ) showed a pronounced rigidity recorded as tonic activity in the electromyogram . 