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: The effects of quinine and 4 - aminopyridine on conditioned place preference and changes in motor activity induced by morphine in rats .
### Output Text:
The effects of <span class="chemical">quinine</span> and <span class="chemical">4 - aminopyridine</span> on conditioned place preference and changes in motor activity induced by <span class="chemical">morphine</span> in rats . 