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: These differences between the sexes could not be related to altered disposition of tetracycline or altered uptake of oleic acid .
### Output Text:
These differences between the sexes could not be related to altered disposition of <span class="chemical">tetracycline</span> or altered uptake of <span class="chemical">oleic acid</span> . 