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: Differences in the parameters of training under the influence of pentobarbital between Groups 1 and 2 were revealed .
### Output Text:
Differences in the parameters of training under the influence of <span class="chemical">pentobarbital</span> between Groups 1 and 2 were revealed . 