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: Significant disfiguring changes occur as a result of bone , cartilage , and soft tissue hypertrophy , including the thickening of the skin , coarsening of facial features , and cutis verticis gyrata .
### Output Text:
Significant disfiguring changes occur as a result of bone , cartilage , and soft tissue hypertrophy , including the thickening of the skin , coarsening of facial features , and cutis verticis gyrata . 