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: However , multiple regression for the daily dosage , duration of phenytoin treatment and cerebellar volume revealed a correlation of these parameters .
### Output Text:
However , multiple regression for the daily dosage , duration of <span class="chemical">phenytoin</span> treatment and cerebellar volume revealed a correlation of these parameters . 