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 were exposed to BCNU on embryonic day 15 and melatonin was given until delivery .
### Output Text:
Rats were exposed to <span class="chemical">BCNU</span> on embryonic day 15 and <span class="chemical">melatonin</span> was given until delivery . 