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: They became more severe in the later months of the experiment , and were most severe after 12 months , located mainly in the molecular layer of the cerebellar cortex .
### Output Text:
They became more severe in the later months of the experiment , and were most severe after 12 months , located mainly in the molecular layer of the cerebellar cortex . 