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: At the 1 . 0 % dosage level , 20 % of rats of both sexes developed neoplastic nodules of the liver , a statistically significant incidence .
### Output Text:
At the 1 . 0 % dosage level , 20 % of rats of both sexes developed neoplastic nodules of the liver , a statistically significant incidence . 