The megaspore-nucleus divides repeatedly, and cells are produced from the peripheral region inwards, which eventually fill the sporecavity with a homogeneous tissue (prothallus); some of the superficial cells at the micropylar end of the megaspore increase in size and divide by a tangential wall into two, an upper cell which gives rise to the short two-celled neck of the archegonium, and a lower cell which develops into a large egg-cell.
By further growth this tissue gives rise to a proembryo, which consists, at the micropylar end, of a sac; the tissue at the chalazal end grows into a long and tangled suspensor, terminating in a mass of cells, which is eventually differentiated into a radicle, plumule and two cotyledons.
M, Micropylar crevice.
This is the first case in which the pollen-bearing organs of a Ptoridosperm have been identified with certainty It will be seen that, while the seeds of Lyginodendron were of an advanced Cycadean type, the microsporangiate organs were more like those of a Fern, the reproductive organs thus showing the same combination of characters which appears in the vegetative ct pc. A, Micropylar region.
Of the three cells at the micropylar end of the sac, all naked cells (the so-called egg-apparatus), one is the egg-cell or oosphere, the other two, which may be regarded as representing abortive egg-cells (in rare cases capable Of fertilization), are known as synergidae.