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: At the same time , the number of heparan sulphate anionic sites and the total anionic site surface ( number of anionic sites x mean anionic site surface ) in the lamina rara externa of the glomerular basement membrane was reduced by 19 % ( p < 0 . 021 ) and by 26 % ( p < 0 . 02 ) , respectively .
### Output Text:
At the same time , the number of <span class="chemical">heparan sulphate</span> anionic sites and the total anionic site surface ( number of anionic sites x mean anionic site surface ) in the lamina rara externa of the glomerular basement membrane was reduced by 19 % ( p < 0 . 021 ) and by 26 % ( p < 0 . 02 ) , respectively . 