Inclusion bodies found in proplastids and immature chloroplasts which possibly play a role in thylakoid formation are described. These include membrane-bound bodies, the bounding membrane of which is continuous with those of the grana and striated osmiophilic droplets whose over-all morphology bears a resemblance to grana stacks. It is suggested that both inclusions contain precursor molecules for membrane synthesis and that they may be actual sites of thylakoid formation.