As the capsule developes, an external layer of cells--the _amphithecium_--and an internal mass--the _endothecium_--are differentiated. As a rule the former becomes the wall of the capsule while the latter gives rise to the spores. In this Division, as in the Pteridophyta, the name _archesporium_ (Fig. 190 _t_) is given to the group of cells inside the sporangium which gives rise to the mother-cells of the spores. The archesporium is in general a unicellular layer; in _Sphagnum_ and _Anthoceros_ it is derived from the most internal layer of the amphithecium, but with these exceptions it arises from the endothecium, usually from its most external layer. In the true Mosses and in _Riccia_ only spore-mother-cells are produced from the archesporium, but in the majority of the Liverworts some of these cells are sterile and become elaters (cells with spirally thickened walls, Figs. 196, 189), or serve as “nurse-cells” for the spore-mother-cells, which gradually absorb the nutriment which has been accumulated in them. In _Anthoceros_, and almost all the Leafy-Mosses, a certain mass of cells in the centre of the sporangium (derived from the endothecium) does not take part in the formation of the archesporium, but forms the so called “column” or “columella” (Figs. 190, 191). The _spores_ arise in _tetrads_, _i.e._ four in each mother-cell, and are arranged at the corners of a tetrahedron, each tetrahedron assuming the form of a sphere or a triangular pyramid. The mature spore is a nucleated mass of protoplasm, with starch or oil as reserve material. The wall is divided into two layers: the external coat (exospore) which is cuticularized and in most cases coloured (brown, yellowish), and the internal coat (endospore), which is colourless and not cuticularized. On germination the exospore is thrown off, the endospore protrudes, and cell-division commences and continues with the growth of the protonema (Fig. 186, _B-D_). [Illustration: FIG. 192.--_Andreæa petrophila._ A ripe sporogonium: _a_ an archegonium which has been raised with the pseudopodium; _p_ the foot; _b_ the neck; _d-e_ the dark-coloured portion of the sporangium, whose outer cell-walls are considerably thickened; _c_-_c_ the thin-walled portions where the dehiscence occurs; _o_ the lower extremity of the spore-sac; _f_ calyptra; _g_ the apex of the sporangium. (Mag. 25 times.)] [Illustration: FIG. 193.--_Andreæa petrophila._ An empty capsule; the calyptra has fallen off. (Mag. 25 times.)] The morphological explanation which Celakovsky has given of the sporogonium, and which is not at all improbable, is, that it is homologous with an embryo consisting of a very small stem-portion and a terminal spore-producing leaf. This will be further explained in the introduction to the Flowering-plants (p. 236).