1. The archesporium of Epipactis consists of a single hypodermal cell, which, without formation of parietals, functions as the megaspore mother cell. 2. In most cases the megaspore mother cell divides to two unequal daughter cells, the chalazal one again dividing to form two megaspores. The innermost megaspore then gives rise to the embryo sac. 3. In other cases four megaspores take part in the formation of the sac, the walls appearing at the first two divisions of the megaspore mother cell being evanescent. At least one of these walls often shows a tendency to persist, which results in a six-nucleate type of sac. The same appearance may also result from irregularities in the orientation of spindles. 4. There is some evidence that the embryo sac may at times be derived from two megaspores. 5. The normal mature embryo sac contains an egg, two synergids, three evanescent antipodal cells, and two polar nuclei which fuse. 6. The usual type of "double fertilization" occurs. 7. The fertilized egg gives rise to an embryo, which, at least in the seed, has no suspensor. 8. The endosperm nucleus, formed by the fusion of one male nucleus with the two polar nuclei, disorganizes without dividing. 9. The variety of methods by which the embryo sac of Epipactis is formed may be regarded as a series representing a reduction in the number of nuclear divisions occurring between the archesporial cell and the formation of the egg. 10. The fate of the nuclei in the different courses of development is probably due to some conditions external to them rather than to any inherent potentialities. A normal sac would probably be produced by any cell subjected to the conditions under which a mother cell would produce one. 11. The sac in its early stages appears to be an equipotential system, polarity being connected with its shape, and the part that the nuclei are to play with their position. 12. The polar groups probably do not represent archegonia, but the general structure of the angiosperm embryo sac may be indicated by some features in those of the gymnosperms.