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: In twenty beagle dogs selective injections were made into the left coronary artery with iopentol , iohexol and metrizoate in doses of 4 ml , 8 ml and 16 ml .
### Output Text:
In twenty beagle dogs selective injections were made into the left coronary artery with <span class="chemical">iopentol</span> , <span class="chemical">iohexol</span> and <span class="chemical">metrizoate</span> in doses of 4 ml , 8 ml and 16 ml . 