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: Dose - response relationships , biochemical mechanisms , and sex differences in the experimental fatty liver induced by tetracycline were studied in the intact rat and with the isolated perfused rat liver in vitro .
### Output Text:
Dose - response relationships , biochemical mechanisms , and sex differences in the experimental fatty liver induced by <span class="chemical">tetracycline</span> were studied in the intact rat and with the isolated perfused rat liver in vitro . 