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: Severe hematologic toxicity ( neutrophil count < 1000 / mm3 and / or hemoglobin < 8 g / dl ) occurred in 4 patients assigned to group I and 7 assigned to group II .
### Output Text:
Severe hematologic toxicity ( neutrophil count < 1000 / mm3 and / or hemoglobin < 8 g / dl ) occurred in 4 patients assigned to group I and 7 assigned to group II . 