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 initial response to the primary attack by the cyclophosphamide metabolites seems to be fragmentation of the luminal membrane .
### Output Text:
The initial response to the primary attack by the <span class="chemical">cyclophosphamide</span> metabolites seems to be fragmentation of the <span class="chemical">luminal</span> membrane . 