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: Rats received subcutaneous injections of 17beta - estradiol ( 2 microg / rat ) or oil once daily for four consecutive days .
### Output Text:
Rats received subcutaneous injections of <span class="chemical">17beta - estradiol</span> ( 2 microg / rat ) or oil once daily for four consecutive days . 