A cross section has examples from all classes.
'Section' is a cut, and 'cross' means 'across'. Imagine you have a gateau made in layers, with cake, chocolate, cream, fruit and so on, one layer on top of the other. If you take a bit from the top, you'll only get what's on the top layer, won't you? But if you take a knife and <em>across </em>the cake, the slice you take out will have a sample of all the layers that the cake is made up of. That's what a 'cross section' is.