The arrangement of the nuclei in a normal embryo sac in the dicot plants is

their are 8 nuclei in embryo sac after the continuous mitosis of functional megaspore (1 nucleus --mitosis--> 2 nuclei --mitosis--> 4 nuclei ---mitosis--> 8 nuclei) 3 nuclei move to the chalazal end ( upper end) & 3 nuclei move to micropylar end (lower end). these six of the eight nuclei are then surrounded by cell walls and organised into cells. the remaining two nuclei, called polar nuclei are situated at the center of embryo sac in the large central cell. 
Thats why there are 8 nuclei and 7 celled embryo sac of normal dicot plant...