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: All three cytopenias were completely reversible following cessation of treatment ; the time required for recovery of the erythron ( approximately 1 month ) was considerably longer than that of the granulocytes and platelets ( hours to a few days ) .
### Output Text:
All three cytopenias were completely reversible following cessation of treatment ; the time required for recovery of the erythron ( approximately 1 month ) was considerably longer than that of the granulocytes and platelets ( hours to a few days ) . 