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: Pregnant Wistar rats were assigned to five groups : intact - control , saline - control , melatonin - treated , BCNU - exposed and BCNU - exposed plus melatonin .
### Output Text:
Pregnant Wistar rats were assigned to five groups : intact - control , saline - control , <span class="chemical">melatonin</span> - treated , <span class="chemical">BCNU</span> - exposed and <span class="chemical">BCNU</span> - exposed plus <span class="chemical">melatonin</span> . 