24 may be coextruded with front wall 14 moreover the front and rear walls 14 16 may be part of a single sheet which is folded along the bottom 26 and sealed along the sides 28 each of these variations may be used without departing from the scope of the present invention as defined in the appended claims referring now more specifically to fig2 the male profile 22 includes a base or web 30 and a projecting element or bulb 32 of substantially cylindrical cross section the bulb 32 has a hollow core 34 which allows the bulb 32 to collapse somewhat when compressed and to return essentially to its original shape when compression is removed the female profile 24 has a groove 36 which is also of substantially cylindrical cross section in accordance with the present invention the cross section of the bulb 32 of the male profile 22 is the same size as or slightly larger than the cross section of the groove 36 of the female profile 24 as a consequence when the bulb 32 is pushed into groove 36 the bulb 32 collapses because of its hollow core 34 once inside the groove 36 however the bulb 32 tends to return to its original shape making intimate contact with the walls of the groove 36 and thereby making a hermetic seal for the bag 10 0021 fig3 is a cross sectional view of zipper 20 shown for the sake of clarity unattached to front and rear walls 14 16 and with male and female profiles 22 24 separated from one another 0022 fig4 is a cross sectional view analogous to that presented in fig3 of an alternate embodiment of the.