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 two groups were then divided into : group 1 , animals receiving saline ; and group 2 , animals receiving aminoguanidine ( AG ) which is a specific inhibitor of inducible - NO synthase .
### Output Text:
These two groups were then divided into : group 1 , animals receiving saline ; and group 2 , animals receiving <span class="chemical">aminoguanidine</span> ( <span class="chemical">AG</span> ) which is a specific inhibitor of inducible - <span class="chemical">NO</span> synthase . 