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: The morphological appearance of the kidneys was examined on days three , four , six , eight and thirty three and the NAG isoenzyme patterns on days zero , four , eight and thirty three .
### Output Text:
The morphological appearance of the kidneys was examined on days three , four , six , eight and thirty three and the NAG isoenzyme patterns on days zero , four , eight and thirty three . 