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: Iron accumulation is considered to be involved in the pathogenesis of Parkinson ' s disease .
### Output Text:
<span class="chemical">Iron</span> accumulation is considered to be involved in the pathogenesis of Parkinson ' s disease . 