At the same time that the embryo is being developed, other changes are taking place in the ovule, especially in the integument which becomes the shell of the seed (_testa_). The endosperm grows, and the embryo-sac supplants the cells of the nucellus. The _seed_ is now formed, and it consists in its most complete development, as in this instance, of three parts: (1) The _testa of the seed_, formed by the enveloping integuments, with the remainder of the tissue of the nucellus lying outside the embryo-sac (the macrosporangium). (2) The _endosperm_ or prothallium. (3) The _embryo_. [Illustration: FIG. 255.--Diagrammatic longitudinal section through an anatropous ovule shortly after fertilisation; _a_ and _i_ are the two integuments; _f_ the funicle; _k_ the nucellus; _S_ the embryo-sac, with the incipient formation of nutritive-tissue; _E_ the embryo; _P_ the pollen-tube passing through the micropyle (_n_) to the oosphere.] The reduction in the ANGIOSPERMS is carried to the extreme limit. In the embryo-sac (the macrospore) the nucleus by continued division produces a prothallium consisting of primordial cells (Fig. 254). In the upper end of the embryo-sac (which is nearest the micropyle) are three cells, two of which are termed the “co-operating cells” (_synergidæ_) and the third is the _oosphere_. Three others are placed at the opposite end of the embryo-sac and are therefore termed the “antipodal cells.” Finally, a large cell is also formed, which occupies the space between the two groups and whose cell-nucleus, the central definitive nucleus, lies in the centre of the embryo-sac. These primordial cells are the slight remnant of the prothallium. The entire structure of the archegonium, with its neck and canal-cells, has disappeared, and nothing is left but the indispensable _oosphere_. When the oosphere has been fertilised, and has commenced the cellular divisions which lead to the formation of the embryo (Fig. 255), the synergidæ and antipodal cells are absorbed, and a cell-formation begins by a new process which emanates from the definitive nucleus and by which a parenchymatous cell-tissue, the nutritive-tissue, arises which may perhaps be considered as homologous with the endosperm of the Gymnosperms. The difference is that the nutritive-tissue of the Angiosperms is formed in two parts with an intervening interruption; the primary nutritive-tissue is first formed, and after fertilisation is absorbed, with the exception of one cell, which continues the development and gives rise to the nutritive-tissue proper, which is formed in the first instance of primordial cells, and later on of a cellular tissue; this nutritive-tissue formed in the embryo-sac is termed “endosperm”; in a few instances[23] a tissue which is derived from the nucellus functions as nutritive-tissue, and is termed “perisperm.” In many plants the seeds, when ripe, contain a very rich nutritive-tissue, in addition to the embryo, for the purpose