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: METHODS : Male adult Wistar rats ( n = 90 and 260 - 290 g ) were divided into 1 , control ; 2 and 3 , crocins ( 15 and 30 mg / kg ) ; 4 , STZ ; 5 and 6 , STZ + crocins ( 15 and 30 mg / kg ) groups .
### Output Text:
METHODS : Male adult Wistar rats ( n = 90 and 260 - 290 g ) were divided into 1 , control ; 2 and 3 , <span class="chemical">crocins</span> ( 15 and 30 mg / kg ) ; 4 , <span class="chemical">STZ</span> ; 5 and 6 , <span class="chemical">STZ</span> + <span class="chemical">crocins</span> ( 15 and 30 mg / kg ) groups . 