Final answer:
There are 9 secondary squares on a hemocytometer grid, and the volume of each secondary square is 0.008 mm³.
Step-by-step explanation:
The diagram of the grid in a Petroff-Hausser chamber, used for counting bacterial cells, shows that there are 9 secondary squares within the main square. These secondary squares are further divided into smaller squares. Each of these smaller squares represents a known area. The volume of each square can be calculated by multiplying its area by the height of the coverslip, which is 0.2 mm. Since the square has an area of 0.04 mm², its volume would be 0.008 mm³. Therefore, the volume of each secondary square is 0.008 mm³.