At the released stage, the residual cell plates and “spore chambers” of the microspore tetrads ofIpomoea purpurea,are the most resistant elements of the callosic special wall. The spore chamber walls show a regular geometric pattern, visible in phase contrast or fluorescence, which constitutes a hollow template or “mould” for the first spore wall or primexine matrix. This matrix forms a network of polygonal meshes at the angles and along the sides of which the probacula and spines of the future exine are embedded until formation of a “foot layer”. In the mature exine, large spines are absent from some of the angles. Their distribution is not random but definite, and is directly related to the primary patterning of the spore chamber. Important developmental stages occur, and the general patterning of the exine is established, whilst the callosic special wall is still present. This indicates that it plays an essential rôle in pollen ontogeny.