Beer . — On the Development of the Spores of Riccia glauc a. 283 
mucilage is always sharply defined from the delicate spore-wall which has 
quite different staining properties, and that it is not difficult to separate the 
one from the other by means of reagents. I regard the mucilage, therefore, 
as a new formation which has no relation either to the special - m other- cell 
wall or to the first spore-wall. 
The older writers believed that this mucilage formed a continuous 
layer over the inner face of the first spore-wall. I do not find this to be the 
case. 
Over the ventral surfaces of older spores, in which the second spore- 
wall has been developed, it can be seen that the two membranes lie closely 
against one another without any mucilage between them (Fig. 21). Over 
the dorsal surface of the spore the two layers of the wall are indeed 
frequently separated from one another and then the space between them 
appears to be, at least partly, occupied by a mucilage. How far this 
separation between the two walls of the spore over the dorsal surface is a 
normal feature of their structure is, however, difficult to say. In older 
spores the mucilage at the equatorial rim has again become absorbed and 
can no longer be seen. 
At first this mucilage is limited internally only by the plasma 
membrane of the protoplast, which is pushed inwards at the equatorial seam 
by the plug of mucilage (Fig. 20). Before long, however, the spore- 
protoplast develops a new wall within the first one, and this then forms a 
flattened internal boundary to the mucilage at the rim of the spore (Fig. 21). 
This second spore-wall is a cuticularized structure almost from the com- 
mencement. At the very first, however, it is probably composed of 
uncuticularized pectose-cellulose, for when it is just discernible it stains more 
deeply with bismarck-brown than is usual with cuticularized membranes, 
and, moreover, the lamellae which are subsequently added to its thickness 
unquestionably have, at first, a pectose-cellulose constitution. 
During the earliest stages of its existence the second spore-wall appears, 
even under high magnifications, as a perfectly homogeneous layer, but in 
spores which are a little older it has become considerably thicker and then 
a dark line can be seen traversing the middle of this wall and dividing it 
into an inner and an outer part (Figs. 22, 35). This dark line grows in 
thickness with the age of the spore until it not infrequently becomes a thick 
layer which stains intensely black with Heidenhain’s haematoxylin and 
forms the most conspicuous feature in the spore-wall (Fig. 26). It is not 
easy to assure oneself of the real nature of this dark layer, but after com- 
paring together a large number of spores I am led to the conclusion that 
this layer most probably only represents a gap which is formed between two 
sets of lamellae and which becomes occupied by some dark-coloured, stain- 
able material. This view seems to me to be supported by the fact that the 
dark layer varies in the time and in the position of its appearance. Some- 
