=654.= If we cut across the compound pistil of trillium we find that the infoldings of the three pistils meet to form three partial partitions which extend nearly to the center, dividing off three spaces. In these spaces are the ovules which are attached to the infolded margins. If we make cross-sections of a pistil of the May-apple (podophyllum) and through the ovules when they are quite young, we shall find that the ovule has a structure like that shown in fig. 389. At _m_ is a cell much larger than the surrounding ones. This is called the macrospore. The tissue surrounding it is called here the nucellus, but because it contains the macrospore it must be the macrosporangium. The two coats or integuments of the ovule are yet short and have not grown out over the end of the nucellus. This macrospore increases in size, forming first a cavity or sac in the nucellus, the _embryo sac_. The nucleus divides several times until eight are formed, four in the micropylar end of the embryo sac and four in the opposite end. In some plants it has been found that one nucleus from each group of four moves toward the middle of the embryo sac. Here they fuse together to form one nucleus, the _endosperm nucleus_ or _definitive nucleus_ shown in fig. 390. One of the nuclei at the micropylar end is the egg, while the two smaller ones nearer the end are the _synergids_. The egg-cell is all that remains of the archegonium in this reduced prothallium. The three nuclei at the lower end are the _antipodal_ cells. [Illustration: Fig. 390. Podophyllum peltatum, ovule containing mature embryo sac; two synergids, and eggs at left, endosperm nucleus in center, three antipodal cells at right.] [Illustration: Fig. 391. Macrospore (one-celled stage) of lilium.] =655. Embryo sac is the young female prothallium.=—In figs. 391-393 are shown the different stages in the development of the embryo sac in lilium. The embryo sac at this stage is the young female prothallium, and the egg is the only remnant of the female sexual organ, the archegonium, in this reduced gametophyte. =656. Fertilization.=—When the pollen tube has reached the embryo sac (paragraph 652) it opens and the two sperm cells are emptied near the egg. The first sperm nucleus enters the protoplasm surrounding the egg nucleus and uniting with the latter brings about fertilization. The second sperm nucleus often unites with the endosperm nucleus (or with one or both of the “polar nuclei”), bringing about what some call a second fertilization. Where this takes place in addition to the union of the first sperm nucleus with the egg nucleus it is called _double fertilization_. The sperm nucleus is usually smaller than the egg nucleus, but often grows to near or quite the size of the egg nucleus before union. See figs. 394 and 395. [Illustration: Fig. 392. Two-and four-celled stage of embryo sac of lilium. The middle one shows division of nuclei to form the four-celled stage. (Easter lily.)]