174 Campbell . — On the Development of 
stained sections contains only very small colourless granules. 
The amount of this finely granular protoplasm increases very 
much in quantity as the prothallium grows. This nucleated 
protoplasm is evidently concerned in the elaboration of the 
reserve food-stuff in the spore, in order to facilitate its absorp- 
tion by the growing prothallium, and later by the embryo. 
These nuclei have a small nucleolus, and are quite rich in 
chromatin. The nuclei usually remain free in the protoplasm, 
but in exceptional cases indications of cell-formation were 
seen, resembling closely the secondary c endosperm 5 in the 
macrospore of Selaginella , and no doubt homologous with it 
(Fig. 53 eti). Nothing of a similar kind is known to exist 
elsewhere ; although, in all probability, a careful examination 
will show the same state of things to obtain in Salvinia. 
That a similar behaviour of the nucleus is not found in the 
Marsiliaceae, may perhaps be explained by the extremely 
rapid development and small size of the prothallium in the 
latter, and the more intimate connexion of the embryo with 
the cavity of the spore. 
The base of the prothallium, which at first is strongly 
convex, gradually becomes straight as the basal cells expand 
laterally (Fig. 49), and later, with the vertical growth of the 
cells, becomes strongly concave, this being especially marked 
in the older prothallia that have remained unfertilized 
(Fig. 60). 
At the time that the first archegonium is ripe, the pro- 
thallium seen in longitudinal section appears nearly hemi- 
spherical, but somewhat narrower at the base owing to the 
lateral growth of the middle cells (Fig. 51). The central cell 
of the archegonium is separated by but one (or occasionally two) 
layers of cells from the spore-cavity, and the neck projects 
considerably above the upper surface of the prothallium. But 
very little chlorophyll is to be seen at this stage, and even in 
the older prothallia but very little is found as compared 
with Salvinia , or with the old unfertilized prothallia of the 
Marsiliaceae. 
As the growing prothallium pushes up, it penetrates the 
