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: Sprague - Dawley rats that were 20 days pregnant and nonpregnant rats of the same age and strain received infusions of aminophylline until onset of maximal seizures which occurred after 28 and 30 minutes respectively .
### Output Text:
Sprague - Dawley rats that were 20 days pregnant and nonpregnant rats of the same age and strain received infusions of <span class="chemical">aminophylline</span> until onset of maximal seizures which occurred after 28 and 30 minutes respectively . 