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: Biochemical and hormonal indices of calcium and bone metabolism were measured throughout the study , and bone biopsies were done at baseline , 60 weeks after OCT or vehicle treatment , and at the end of the crossover period .
### Output Text:
Biochemical and hormonal indices of <span class="chemical">calcium</span> and bone metabolism were measured throughout the study , and bone biopsies were done at baseline , 60 weeks after <span class="chemical">OCT</span> or vehicle treatment , and at the end of the crossover period . 