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: The central nervous system toxicity of high - dose cytosine arabinoside is well recognized , but the toxicity of cytosine arabinoside in the peripheral nervous system has been infrequently reported .
### Output Text:
The central nervous system toxicity of high - dose <span class="chemical">cytosine arabinoside</span> is well recognized , but the toxicity of <span class="chemical">cytosine arabinoside</span> in the peripheral nervous system has been infrequently reported . 