TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHUS. 17 
enamel is being formed, much shorter elsewhere (Tomes, 1. c., 
p. 112), They are shown bordering the enamel in PI. Ill, 
fig. 1, e. c. Tomes’s processes are seen projecting from the 
inner ends of the cells when torn away from the enamel. The 
layer is somewhat sharply marked off from the stratum inter- 
medium. In preparing the dissection shown in PI. II, fig. 16 
it was noticed that these cells adhered firmly to the calcified 
part of the tooth, although they were easily separated else- 
where. The same fact is indicated in many of the figures (1 
to 15 on PI. II). 
5. The Stratum Intermedium of Hannover. — Of 
entirely normal structure and appearance (see PI. Ill, fig. 
1, s. i.). I could not detect capillaries in the layer, as 
affirmed by Lionel Beale, although they are certainly present 
in the stellate reticulum, and are sometimes seen very near 
this layer. Injected specimens would be necessary in order to 
be quite certain of their absence. 
6. The Middle Membrane of the Enamel Organ — the 
Stellate Beticulum. — This layer is largely developed, and 
gives to the young teeth a very characteristically mammalian 
appearance. The extent of the layer, and in fact the relative 
distribution and thickness of nearly all the layers, is best seen 
in figs. 1 to 15 (PI. II). The details of the layer are shown 
in PI. Ill, figs. 1, 4, 5, and 6 ( m . m. in all figures). It is quite 
certain that blood-vessels are present in this layer, and that 
they extend into all parts of it. The presence of blood-vessels 
in the mammalian enamel organ has been affirmed and denied 
(Tomes, 1. c., p. 127). Klein also states that blood-vessels 
are not present in the middle membrane (‘Atlas of Histology/ 
p. 185). I have, however, examined some beautiful sections 
of developing teeth in the rat kindly lent me by Professor 
Howes, and there is certainly no doubt about the presence of 
abundant blood-vessels in this layer, in which they had been 
previously noticed by Professor Howes. In many cases altered 
blood-corpuscles remained in the lumen of vessels in very large 
numbers. It is very extraordinary that the existence of such 
obvious vascular channels should have been denied. I propose 
VOL. XXIX, PART 1. NEW SER. 
B 
