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: There was a marked increase in the number of TUNEL positive cells and nestin positive cells in BCNU - exposed group , but a decreased immunoreactivity to glial fibrillary acidic protein , synaptophysin and transforming growth factor beta1 was observed , indicating a delayed maturation , and melatonin significantly reversed these changes .
### Output Text:
There was a marked increase in the number of TUNEL positive cells and nestin positive cells in <span class="chemical">BCNU</span> - exposed group , but a decreased immunoreactivity to glial fibrillary acidic protein , synaptophysin and transforming growth factor beta1 was observed , indicating a delayed maturation , and <span class="chemical">melatonin</span> significantly reversed these changes . 