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: Pregnant rats were administered one of these calcium channel blockers during the period of cardiac morphogenesis and the offspring examined on day 20 of gestation for cardiovascular malformations .
### Output Text:
Pregnant rats were administered one of these <span class="chemical">calcium</span> channel blockers during the period of cardiac morphogenesis and the offspring examined on day 20 of gestation for cardiovascular malformations . 