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: Anti - carcinogenic action of phenobarbital given simultaneously with diethylnitrosamine in the rat .
### Output Text:
Anti - carcinogenic action of <span class="chemical">phenobarbital</span> given simultaneously with <span class="chemical">diethylnitrosamine</span> in the rat . 