Pasteur observed that the broth in the straight-neck flask was discolored and cloudy, while the broth in the curved-neck flask had not changed.
Explanation:
He concluded that germs in the air were able to fall unobstructed down the straight-necked flask and contaminate the broth. The other flask, however, trapped germs in its curved neck, preventing them from reaching the broth, which never changed color or became cloudy.
The nucleus is considered the command center of the cell due to the fact that it contains most of the genetic material that carries the cells instructions for the cells operations.