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: We investigated the effect of an early stress , in the form of maternal deprivation , combined with a later stress , simulated by chronic periadolescent corticosterone treatment , on behaviour in rats .
### Output Text:
We investigated the effect of an early stress , in the form of maternal deprivation , combined with a later stress , simulated by chronic periadolescent <span class="chemical">corticosterone</span> treatment , on behaviour in rats . 