i did what you said. i left just a quarter of a poly sphere, just like the outside of the slice and then selected the edges of one side, one by one so they made half a circle. the problem is when i extruded. it does close the gap somehow, but it makes a bump in the center and the line that is created is not aligned. i just extruded on one axis, being careful not to spoil the geometry. what is wrong? if it helps you, when i extrude it closes slowly not to a straight line but like a circle that is closing and with a bump on the front-back axis..

i also did the append poly tool. it just returned the scene to object mode. what is the purpose of that tool?

thanks in advance.