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: Serotonin and N - acetyl serotonin which augmented ketamine DOC , did not do so in the absence of the pineal gland , whereas melatonin potentiated the ketamine DOC in both the intact and pinealectomized chick .
### Output Text:
<span class="chemical">Serotonin</span> and <span class="chemical">N - acetyl serotonin</span> which augmented <span class="chemical">ketamine</span> DOC , did not do so in the absence of the pineal gland , whereas <span class="chemical">melatonin</span> potentiated the <span class="chemical">ketamine</span> DOC in both the intact and pinealectomized chick . 