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: Prolactin levels were measured and the uteri of the rats were removed for histopathological analysis at the end of 98 days .
### Output Text:
Prolactin levels were measured and the uteri of the rats were removed for histopathological analysis at the end of 98 days . 