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: Side effects of postoperative administration of methylprednisolone and gentamicin into the posterior sub - Tenon ' s space .
### Output Text:
Side effects of postoperative administration of <span class="chemical">methylprednisolone</span> and <span class="chemical">gentamicin</span> into the posterior sub - Tenon ' s space . 