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### Input Text: Rats treated for 11 days with morphine and withdrawn for 36 - 40 h showed differences in the development of tolerance : about half of the animals showed a rigidity after the test dose of morphine that was not significantly less than in the controls and were akinetic ( A group ) .
### Output Text:
Rats treated for 11 days with <span class="chemical">morphine</span> and withdrawn for 36 - 40 h showed differences in the development of tolerance : about half of the animals showed a rigidity after the test dose of <span class="chemical">morphine</span> that was not significantly less than in the controls and were akinetic ( A group ) . 