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 restimulation of this area with N - methyl - D - aspartic acid 15 days postlesion failed to produce a pressor response .
### Output Text:
The restimulation of this area with <span class="chemical">N - methyl - D - aspartic acid</span> 15 days postlesion failed to produce a pressor response . 