Fig. 1 (Stage 5). Tassel Development Stages:

1. Tassel visible
2. Peduncle visible
3. Fully expanded
4. First pollen shed (main spike)
5. Shed from branches
6. Shed from >1/2 branches
7. Shed from all branches
8. Pollen shed complete
9. Seven days after shed complete

Stage 4 represents the beginning of pollen shed with anthers exposed on the main tassel branch. Stage 8 is the end of pollen shed; no new anthers are exposed. Arrows on Stage 4 delimit the length of the main branch. Main stem diameter was measured at the lower arrow position.

Image from Crop Management article:
Tassel Morphology as an Indicator of Potential Pollen Production in Maize