Abstract: Disclosed herein are methods, assemblies, systems, kits and devices for introducing molecules or compositions into cells or cell-like bodies. An assembly for introducing molecules in a solution into cells or cell-like bodies comprises a rigid container having a first inner diameter or cross-sectional area at a proximal end thereof and inner and outer walls extending between a distal and proximal end, a plunger insertable into the container at the proximal end, and at least one constriction of only the inner wall proximal to the distal end or at least one constriction of the inner and the outer walls proximal to the distal end, wherein the at least one constriction has a second inner diameter or cross-sectional area that is smaller than the container first inner diameter or cross-sectional area and the plunger is axially movable along the container.