The method of cell-division in these spores resembles that known as “cleavage by constriction,” in which the cross wall arises by an ingrowth from all sides of the cell; in time the centre is reached and the wall is complete, or an open pore is left between the divided cells. Cell “cleavage” occurs frequently among Thallophytes, though it is unknown among the higher plants. Among Algae it is the normal form of cell-division in _Cladophora_ and also in _Spirogyra_, though in the latter the wall passes right across and cuts through the connecting plasma threads. Harper[684] found “cleavage by constriction” in two instances among fungi: the conidia of _Erysiphe_ and the gametes of _Sporodinia_ are cut off by a septum which originates as a circular ingrowth of the outer wall, comparable, he considers, with the cell-division of _Cladophora_. The development of the thickened wall of polarilocular spores has been studied by Hue[685], who contends however that there is no true septation in the colourless spores so long as the central canal remains open. According to his observations the wall of the young spore is formed of a thin tegument, everywhere equal in thickness, and consisting of concentric layers. This tegument becomes continually thicker at the equator of the spore by the addition of new layers from the interior, and the protoplasmic contents are compressed into a gradually diminishing space. In the end the wall almost touches at the centre, and the spore consists of two polar cell-cavities with a narrow open passage between. A median line pierced by the canal is frequently seen. In a few species there is a second constriction cleavage and the spore becomes quadrilocular. Hue insists that this spore should be regarded as only one-celled; for though the walls may touch at the centre, he says they never coalesce. He has unfortunately given no cytological observations as to whether the spore is uni- or binucleate. In _Xanthoria parietina_, one of the species with characteristic polaribilocular spores, germination, it would seem, takes place mostly at one end only of the spore, though a germinating tube issues at both ends frequently enough to suggest that the spore is binucleate and two-celled. The absence of germination from one or other of the cells only may probably be due to the drain on their small resources. Hue has cited the rarity of such instances of double germination in support of his view of the one-celled nature of the spore. He instances that out of fifteen spores, Tulasne[686] has figured only three that have germinated at each end; Bornet[687] figures one in seven with the double germination and Bonnier[688] one in sixteen spores.