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: Ketamine was more potent in the hypophysectomized chick and the circadian rhythm noted in the intact chick was absent ; furthermore , melatonin did not augment the ketamine DOC whereas dopamine continued to do so .
### Output Text:
<span class="chemical">Ketamine</span> was more potent in the hypophysectomized chick and the circadian rhythm noted in the intact chick was absent ; furthermore , <span class="chemical">melatonin</span> did not augment the <span class="chemical">ketamine</span> DOC whereas <span class="chemical">dopamine</span> continued to do so . 