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: AIM : To study the effects of testosterone on streptozotocin ( STZ ) - induced memory impairment in male rats .
### Output Text:
AIM : To study the effects of <span class="chemical">testosterone</span> on <span class="chemical">streptozotocin</span> ( <span class="chemical">STZ</span> ) - induced memory impairment in male rats . 