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: All children who experienced seizures survived with good graft function and remained seizure - free without anti - epileptic drugs over a mean follow - up period of 33 . 7 + 14 . 6 months .
### Output Text:
All children who experienced seizures survived with good graft function and remained seizure - free without anti - epileptic drugs over a mean follow - up period of 33 . 7 + 14 . 6 months . 