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: Cross - sectional echocardiography was used to evaluate two neonates whose mothers ingested lithium during pregnancy .
### Output Text:
Cross - sectional echocardiography was used to evaluate two neonates whose mothers ingested <span class="chemical">lithium</span> during pregnancy . 