471 
and in the mode of its development, it shows a still greater resemblance to the tissue 
that fills up the embryo-sac of Gymnosperms, and even of Angiosperms. In Isoetes 
the cavity begins to be filled with cellular tissue a few weeks after the escape of the 
macrospores from the decaying macrosporangium ; the cells of this tissue are all at 
first naked (without cell-wall); they appear to become enclosed in firm cell-walls 
only when the whole cavity of the endospore is filled with them (Fig. 330). In 
the meantime the endospore thickens, becomes differentiated into layers, and 
assumes a finely granular appearance, phenomena which, as Hofmeister insists, are 
exhibited in like manner in the embryo-sacs of Coniferae. The spherical pro- 
m 
Fig. 331.— Germination of Selaginella (after Pfeffer); I— III, S. Martensii, A—'O, S. caiilescens ; / longitudinal section 
of a macrospore filled with the prothallium and ' endosperm,' d the diaphragm, e e' two embryos in process of formation ; 
// a young archegonium not yet open; /// an archegonium with the oospore fertilised and divided once ; A a microspore 
showing the primordial cells ; B C different views of these divisions ; D the mother-cells of the antherozoids in the perfect 
antheridium ; v, vegetative cell. 
thallium now swells up, the three converq:ent edges of the exospore burst length- 
wise and thus form a three-rayed fissure, where the prothallium is covered only 
by the membranous endospore; this also peels off, and softens, finally exposing 
the corresponding part of the prothallium. At its apex appears the first arche- 
gonium ; if this is not fertilised, several others are subsequently formed at its side. 
In Selaginella, even when the macrospores are still lying in the sporangium, the 
apical region is found to be clothed with a small-celled meniscus-shaped mass of 
tissue which is probably formed, during the ripening of the spores, by the division 
