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: In STZ + crocin animal groups , crocin was applied in doses of 15 and 30 mg / kg , i . p . , one day pre - surgery and continued for three weeks .
### Output Text:
In <span class="chemical">STZ</span> + <span class="chemical">crocin</span> animal groups , <span class="chemical">crocin</span> was applied in doses of 15 and 30 mg / kg , i . p . , one day pre - surgery and continued for three weeks . 