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: During a transient ( 20 - s ) coronary occlusion , a perfusion defect was observed with contrast echocardiography in 14 of the 15 dogs in which the occlusion was produced .
### Output Text:
During a transient ( 20 - s ) coronary occlusion , a perfusion defect was observed with contrast echocardiography in 14 of the 15 dogs in which the occlusion was produced . 