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: This was accounted for by a significant number of depressions occurring in methyl dopa treated patients with psychiatric histories .
### Output Text:
This was accounted for by a significant number of depressions occurring in <span class="chemical">methyl dopa</span> treated patients with psychiatric histories . 