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: Within 8 hours after initiation of therapy the patient died with a clinical picture resembling massive pulmonary obstruction due to choriocarcinomic tissue plugs , probably originating from the uterus .
### Output Text:
Within 8 hours after initiation of therapy the patient died with a clinical picture resembling massive pulmonary obstruction due to choriocarcinomic tissue plugs , probably originating from the uterus . 