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: Of 67 patients initially enrolled , 24 remained on thalidomide for 3 months , 8 remained at 6 months , and 3 remained at 9 months .
### Output Text:
Of 67 patients initially enrolled , 24 remained on <span class="chemical">thalidomide</span> for 3 months , 8 remained at 6 months , and 3 remained at 9 months . 