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: Formation of these plugs was probably due to extensive tumor necrosis at the level of the walls of the major uterine veins , which resulted in an open exchange of tumor plugs to the vascular spaces ; decrease in tumor tissue coherence secondary to chemotherapy may have further contributed to the formation of tumor emboli .
### Output Text:
Formation of these plugs was probably due to extensive tumor necrosis at the level of the walls of the major uterine veins , which resulted in an open exchange of tumor plugs to the vascular spaces ; decrease in tumor tissue coherence secondary to chemotherapy may have further contributed to the formation of tumor emboli . 