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: Time dependence of plasma malondialdehyde , oxypurines , and nucleosides during incomplete cerebral ischemia in the rat .
### Output Text:
Time dependence of plasma <span class="chemical">malondialdehyde</span> , <span class="chemical">oxypurines</span> , and <span class="chemical">nucleosides</span> during incomplete cerebral ischemia in the rat . 