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: Chronic administration of estrogen to the Fischer 344 ( F344 ) rat induces growth of large , hemorrhagic pituitary tumors .
### Output Text:
Chronic administration of <span class="chemical">estrogen</span> to the Fischer 344 ( F344 ) rat induces growth of large , hemorrhagic pituitary tumors . 