Why is an egg cell smooth and spherical?

smoothness :
they seem smooth but actually they are not smooth but on microscopic view they show small pores and these pores are formed after embryo formation for exchange of gases ,, 
shape:
they may be spherical , sub spherical and elongate ,, to reduce pressure by increasing their surface area ,, also to prevent rolling off from any upper place .. also it depends upon the mother's reproductive morphology..

Regular or uniform type