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: Although low concentrations ( less than or equal to 50 ppm ) of corticosterone had beneficial effects on TOTP - induced neuropathy , greater than or equal to 200 ppm exacerbated clinical signs in chickens given either TOTP or DFP .
### Output Text:
Although low concentrations ( less than or equal to 50 ppm ) of <span class="chemical">corticosterone</span> had beneficial effects on <span class="chemical">TOTP</span> - induced neuropathy , greater than or equal to 200 ppm exacerbated clinical signs in chickens given either <span class="chemical">TOTP</span> or <span class="chemical">DFP</span> . 