TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 47 
and shows that the columns of soft cells occur isolated, and also arranged in 
small groups. The concentric arrangement of cells round the columns is 
indicated, and the corneous cells which make up the hard part of the structure 
are represented by their darker central portions (better shown in the next 
figure). 
Fig. 8. — X 405. A single column of moderate size from the inner part 
of the section drawn in Fig. 6. The column is seen in transverse section 
surrounded by concentric cells, and these again by the matrix of corneous 
cells. The column itself has stained deeply, especially the central cell, while 
the concentric cells stain faintly and the corneous cells remain unstained. 
Each of the latter contains a central mass of granular appearance, and con- 
taining minute pigment granules. It probably represents the remains of the 
nucleus, together with some of the granular material which occupies a much 
larger space in many softer cells (compare the concentric cells of this section 
and various cells in Fig. 11). This central portion remains comparatively 
soft and dries up in ground sections, being replaced by air. 
Fig. 9. — X 188. A horizontal section through the stratum corneum of the 
concavity of one of the posterior horny plates. The section was ground 
down, dried, and mounted in balsam, and the latter medium has displaced the 
air from most of the larger spaces caused by the shrinkage of the columns 
and the majority of concentric cells. Some of these latter, however, retain 
abundant air and appear dark, and the same is true of the centres of the 
corneous cells. In other cases the air may remain in the shrunken columns, 
so that the resemblance to the Haversian systems of bone would be even more 
striking than in the figure. 
Fig. 10. — X 188. A part of Fig. 4, more highly magnified. The figure 
represents a vertical section through the superficial stratum corneum of the 
outer slope of the plate. Two columns of soft cells arc seen rising to the 
surface through the matrix of corneous cells. 
Fig. 11. — X 188. A vertical section through the lower part of one of the 
posterior upper plates, including the bone. The space between the plate and 
the bone is seen to be very narrow, when the magnification is taken into 
account. This appeared to be the case in all the sections of the posterior 
plates. One of the long thin papillae is seen together with the base of 
another. The stratum Malpighii is of normal appearance : at its upper part 
the cells become granular, and higher still become corneous peripherally. 
Above this we enter the stratum corneum, where the cells are more flattened 
and become almost entirely cornified. Nevertheless many cells occur, 
especially in the lower part of this layer, in which the thickened border is 
alone corneous, while the central part remains granular and is coloured by 
carmine. At a higher level than that shown in the figure the nucleus ceases 
to be distinct, but a central granular pigmented mass remains (compare 
Fig. 8). The soft cells of the column which rises from the apex of the papillae 
