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: Conventional LW rats , implanted with testosterone at age 4 months , developed a higher incidence of prostate cancer after an average interval of 14 months : 24 % had developed gross tumors , and 40 % when it included microscopic tumors .
### Output Text:
Conventional LW rats , implanted with <span class="chemical">testosterone</span> at age 4 months , developed a higher incidence of prostate cancer after an average interval of 14 months : 24 % had developed gross tumors , and 40 % when it included microscopic tumors . 