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: Radiolabelling , following the in vivo injection of 3H - 17 beta estradiol , was increased only over the nuclei of tumor cells ; stereologic analysis revealed a 4 . 5 - to 6 . 7 - times higher concentration of reduced silver grains over nuclei than cytoplasm of these cells .
### Output Text:
Radiolabelling , following the in vivo injection of 3H - 17 beta <span class="chemical">estradiol</span> , was increased only over the nuclei of tumor cells ; stereologic analysis revealed a 4 . 5 - to 6 . 7 - times higher concentration of reduced <span class="chemical">silver</span> grains over nuclei than cytoplasm of these cells . 