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: Upon rechallenge with either cephalosporin , the hematologic syndrome was reproduced in most dogs tested ; cefonicid ( but not cefazedone ) - treated dogs showed a substantially reduced induction period ( 15 + / - 5 days ) compared to that of the first exposure to the drug ( 61 + / - 24 days ) .
### Output Text:
Upon rechallenge with either <span class="chemical">cephalosporin</span> , the hematologic syndrome was reproduced in most dogs tested ; <span class="chemical">cefonicid</span> ( but not <span class="chemical">cefazedone</span> ) - treated dogs showed a substantially reduced induction period ( 15 + / - 5 days ) compared to that of the first exposure to the drug ( 61 + / - 24 days ) . 