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: Cephalosporin antibiotics cause a variety of hematologic disturbances in man , the pathogeneses and hematopathology of which remain poorly characterized .
### Output Text:
<span class="chemical">Cephalosporin</span> antibiotics cause a variety of hematologic disturbances in man , the pathogeneses and hematopathology of which remain poorly characterized . 