One day the Buddha was walking in the forest with Ananda, his closest disciple, and Ananda said to the Buddha, "Lord, have you told us everything that you have to teach us or are there things which you know which you have not taught." The Buddha held out his hands full of leaves and said,"The leaves which you see in the Blessed One's hands are the things which I teach, and the leaves you see spread on the forest floor are the things I know which I do not teach."
Here is the meaning of this: If you hold a green leaf up to the sunlight you can see that the leaf is made of individual cells. The Buddha would have known this. He could have inferred that the bodies of all living things are made of cells, but he did not teach this because it would have confused people.