▁( le aving ▁behind ▁only ▁a ▁few ▁nuc lei , ▁which ▁will ▁become ▁the ▁y olk ▁nuc lei ). ▁After ▁the ▁ 1 0 th ▁division , ▁the ▁pole ▁cells ▁form ▁at ▁the ▁posterior ▁end ▁of ▁the ▁emb ry o , ▁seg reg ating ▁the ▁germ ▁line ▁from ▁the ▁syn cy t ium . ▁Finally , ▁after ▁the ▁ 1 3 th ▁division , ▁cell ▁memb ran es ▁slowly ▁inv ag inate , ▁div iding ▁the ▁syn cy t ium ▁into ▁individual ▁som atic ▁cells . ▁Once ▁this ▁process ▁is ▁completed , ▁g astr ulation ▁starts . ▁ ▁N uc lear ▁division ▁in ▁the ▁early ▁D ros oph ila ▁emb ry o ▁happens ▁so ▁quickly , ▁no ▁proper ▁check points ▁exist , ▁so ▁mistakes ▁may ▁be ▁made ▁in ▁division ▁of ▁the ▁DNA . ▁To ▁get ▁around ▁this ▁problem , ▁the ▁nuc lei ▁that ▁have ▁made ▁a ▁mistake ▁det ach ▁from ▁their ▁cent ros om es ▁and ▁fall ▁into ▁the ▁centre ▁of ▁the ▁emb ry o ▁( y olk ▁sac ), ▁which ▁will ▁not ▁form ▁part ▁of ▁the ▁fly . ▁ ▁The ▁gene ▁network ▁( trans cription al ▁and ▁protein ▁interactions ) ▁gover ning ▁the ▁early ▁development ▁of ▁the ▁fruit ▁fly ▁emb ry o ▁is ▁one ▁of ▁the ▁best ▁understood ▁gene ▁networks ▁to ▁date , ▁especially ▁the ▁pattern ing ▁along ▁the ▁an ter op oster ior ▁( AP ) ▁and ▁dor so vent ral ▁( D V ) ▁axes ▁( See ▁under ▁morph ogen esis ). ▁ ▁The ▁emb ry o ▁under go es