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: Resting irCRF levels are suppressed only at the highest corticosterone infusion rate , which resulted in systemic corticosterone levels of 40 micrograms / dl .
### Output Text:
Resting irCRF levels are suppressed only at the highest <span class="chemical">corticosterone</span> infusion rate , which resulted in systemic <span class="chemical">corticosterone</span> levels of 40 micrograms / dl . 