The role of infinitely small particles in Cartesian physics
In Cartesian physics the rejection of the atomism is the consequence of denying the vacuum. The motion of finitely small and rigid bodies in the space without vacuum can only be limited. The multifarious and variable motion assumed by mechanical philosophy is not possible due to geometrical restraints. Therefore the change of the physical bodies or sets of bodies is indispensable.
If we suppose that in Cartesian physics the shape and size of primary bodies is constant, chaotic motion is only possible if a certain proportion of the bodies are indefinitely small as opposed to finitely small atoms. These indefinitely small particles build up sets of bodies able to change their shape within the gaps between larger bodies.
Consequently, rejecting atomism and assuming indefinite divisibility by Descartes does not mean that primary bodies change their shape and size as part of the physical processes but only that there exist indefinitely small particles besides the finitely small bodies that are similar to atoms.