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: As observed with different doses , the magnitude of induction in the activity of P450 monooxygenases was several fold higher in liver microsomes when compared with the brain .
### Output Text:
As observed with different doses , the magnitude of induction in the activity of P450 monooxygenases was several fold higher in liver microsomes when compared with the brain . 