1907] CAROTHERS—GINKGO BILOBA 123 
been formed and the central cell has enlarged considerably before the 
tissue lining the sac equals in depth the width of the central cavity 
(fig. 37). Htrasé (4) has described the development of the arche- 
gonia. 
The outer wall of the prothallium, the first wall built by the plas- 
mic sac, becomes much thickened, and the megaspore membrane or 
embryo sac becomes exceedingly thick and dense, making the entrance 
of the killing and the fixing fluids difficult. The structure and develop- 
ment of the megaspore wall is set forth in a paper by THOMSON (11), 
but as my results differ slightly from his, I shall givethem. 'THoMsoN 
states the thickness of the megaspore coat from a mature seed to be 
4.5-5#. The wall shown in fig. 22 measures 2.1 » in thickness and 
that of an endosperm which has just closed in the center measures 
6.16. At first the membrane, which as shown above is in reality the 
mother cell wall, is thin and delicately granular (fig. 20). It rapidly 
thickens and becomes coarsely granular. A cross-section shows a 
very thin inner layer acting as a base for the transversely placed rods 
of the thick outer layer (fig. 22). THomson reports these rods 
as being quite irregularly placed. Subsequent thickening takes place 
in the outer layer only, the rods increasing in length and thickness, 
and the thin inner layer finally disappearing (fig. 25). Not infre- 
quently, in material of this stage, little bunches of the rods forming 
the wall are found torn out and scattered over the slide. The result 
of the ease with which the outer layer is torn across between the rods 
and the resistance offered by the thin inner layer is shown at the upper 
end of the wall in jig. 22. The ends of the rods, presented at right 
angles at the surface of the spore coat, are the cause of the slight 
roughness there. After the disappearance of the smooth inner layer, 
the inner surface becomes similarly rough. 
The complete independence of the heavy embryo sac and the 
outer wall of the prothallium explains the position of the former with 
reference to the archegonial chamber of the’ mature endosperm. 
Being entirely free from the tissue of the endosperm, it is lifted up by 
the growth of the tissue around the archegonial chamber until it is 
some distance from the floor of the latter and forms a roof over it. 
From early in May the ovules are green throughout, but as they 
increase in size the inner tissue becomes hyaline, the chlorophyll being 
