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: Conversion was early in 29 patients ( 41 . 4 % ) and late in 41 ( 58 . 6 % ) .
### Output Text:
Conversion was early in 29 patients ( 41 . 4 % ) and late in 41 ( 58 . 6 % ) . 