Gametophyte of Selaginella . 425 
a single superficial layer of small cells, beneath which, in the 
central portion, may usually be found two others. At the 
margins the disc is reduced to a single layer. As the spore 
ripens, the walls of the lower cells become much thickened 
and form the diaphragm ( d ), separating the prothallia disc 
from the thick layer of protoplasm which lines the spore 
cavity (Fig. 11). 
The protoplasmic layer thickens until the vacuole, which at 
first occupies very much the larger part of the spore cavity, 
becomes very much reduced in size (Fig. 10), but at the time 
that the spore is ripe it is still evident. Neither at this time 
nor at an earlier stage was there found in the microtome 
sections any trace of oil or any solid matter. If oil is present 
in the living spore, it was entirely dissolved out in the process 
of embedding. Some time after germination begins the 
vacuole closes completely, and in a few preparations, before 
this was completed, the vacuole showed a spongy-looking 
mass filling it. 
In the later stages the nuclei lying in the cytoplasm 
immediately below the diaphragm (Fig. 12) are much smaller 
than those in the basal part of the spore (Fig. 13), and much 
more numerous. The cytoplasm below the diaphragm is 
more finely granular than at the base of the spore ; and every- 
thing indicates that it is more active, which is what might be 
expected, as it is here that the secondary cell-formation begins 
after the fertilization of the archegonium. 
In the later stages of germination the development of 
coarse masses of proteid materials which stain deeply makes 
the study of the nuclei difficult, and they are by no means 
easy to demonstrate at this time. 
Comparing Miss Lyon’s account of the gametophyte of 
S. apus with that of N. Kraussiana , the first difference is the 
much greater development of the primary prothallium of the 
former, which consists of six to seven layers of cells instead of 
the three usually occurring in .S. Kraussiana . To judge from 
the figures, there are two types of prothallium in N. apus , one 
in which there is a sharp demarcation of the base of the 
