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: Migraine ( 20 % ) was not an uncommon cause of cranial neuropathy although malignancies arising from the reticuloendothelial system or related structures of the head and neck were more frequent ( 26 % ) .
### Output Text:
Migraine ( 20 % ) was not an uncommon cause of cranial neuropathy although malignancies arising from the reticuloendothelial system or related structures of the head and neck were more frequent ( 26 % ) . 