ONTOGENY OF THE PALEOZOIC OVULE,CALLOSPERMARION PUSILLUM

The ontogeny of the upper Pennsylvanian age gymnospermous ovule, Callospermarion pusillum, is described from petrifaction specimens collected at the Berryville locality in Illinois. Ovules exhibit a wide range of dimensional and structural features that indicate an extensive developmental sequence. Specimens range from ovules with indistinct zonation of the thin-walled integument to those with thick-walled cells of the sclerotesta. The apex of the fleshy nucellus in some specimens is preserved as a cellular mound, while in others a well-formed cellular pollen chamber is present; still other ovules are characterized by a papery-thin nucellus and pollen chamber wall. The megagametophyte of most specimens is represented by a hollow megaspore membrane that may be restricted to the base of the nucellus, or occupy the entire seed cavity. In a few specimens cellular gametophytes are preserved, and in one ovule archegonia with supposed eggs are also present. Variability in each of the features is compared with ontogenetic changes in comparable structures of living gymnospermous ovules and is correlated with ovule size. A developmental sequence for the fossil ovules is proposed.