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: Tolerability of lomustine in combination with cyclophosphamide in dogs with lymphoma .
### Output Text:
Tolerability of <span class="chemical">lomustine</span> in combination with <span class="chemical">cyclophosphamide</span> in dogs with lymphoma . 