OP THE ORNTTHORHYNCHUS PLATYPUS. 
273 
A reticulate membrane covers and is closely adherent to the upper surface of the 
rod heads, and extends outwards and inwards over the summits of the bristle cells—- 
the bristles passing through rounded meshes of the membrane. This membrana 
reticularis is made up of one row of circular meshes (annulse) to the outer side of the 
rods, and on the inner side three alternating rows of annulse with elongated meshes 
(phalanges) between. 
Passing downwards from the inner portion of the membrana reticularis to the 
membrana basilaris we find three or four rows of fibres (trabeculae), which run almost 
parallel to the inner rods, and are attached to the latter membrane by enlarged feet 
very like those of the rods. Between these trabeculae and below the level of the 
bristle cells are three or four rows of nuclear cells—that is to say, cells with well- 
marked nuclei, but apparently no cell-walls; these are the cells of Deiters. 
In each of the lower angles of the triangular tunnel is situated a row of nuclear 
cells similar to the cells of Deiters, and to the outer side of the outer rods are a 
number of these nuclear cells lying on the under lip of the sulcus. 
The inner and outer boundary of the organ is formed of irregular columnar cells 
(the supporting cells), which are the transitional link between the ordinary epithelial 
layer and the nerve epithelium proper. 
The nerve filaments which come through a row of holes on the upper surface of the 
lower lip of the sulcus (habenula perforata), pass into the organ of Corti, and no 
doubt terminate in the nuclear cells and bristle cells ; but this point I have not been 
able to demonstrate in the Ornithorhynchus. 
To complete the description of the ductus cochlese there only remain its commence¬ 
ment and termination to be considered. The former I have not yet been able to trace 
out, but with the latter I have been more fortunate. 
The organ of Corti and the rest of the lamina membranacea is not continued further 
forwards than the end of the lamina ossea; but the ductus does not terminate here, 
for it is continued round the circumference of the spheroidal extremity of the bony 
tube, and ends in an enlarged cul-de-sac close to the base of the lamina ossea. Thus 
the ductus makes a turn of about three-quarters of a complete circle at the apex of 
the cochlea tube. This will be better understood by referring to the drawing (Plate 
45, fig. 1), where it will be seen that the ductus almost completely surrounds the oval 
communication (helicotrema) between the scalse tympani and vestibuli. 
At the end of the lamina ossea, where the ductus commences to curve round, a 
complete change takes place in the shape and walls of the membranous labyrinth. It 
first becomes contracted into a circular tube, and after passing the extreme apex of the 
cochlea it enlarges again into an ovoid chamber—the lagena (see Plates 45, 46, figs. 4, 5). 
The walls of this chamber are formed by a thick layer of dense connective tissue, 
lined internally by a single layer of epithelium. The cells of this epithelial layer are 
for the most part cuboid, but near the further end of the lagena a patch of nerve 
