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: In view of the close time association between the start of chemotherapy and the acute onset of massive embolism other explanations , such as spontaneous necrosis , must be considered less likely .
### Output Text:
In view of the close time association between the start of chemotherapy and the acute onset of massive embolism other explanations , such as spontaneous necrosis , must be considered less likely . 