44 
EDWARD B. POULTON. 
organ is seen at m. m. It is of normal structure, except that it certainly 
contains abundant blood-vessels. The vascularity of the enamel organ has 
been frequently affirmed and denied by various observers, but there can be 
no doubt about the question in Ornithorhyuchus. A normal capillary is seen 
in the tooth-sac at c., and it can be traced deeply into the “ stellate reticu- 
lum ” of the enamel organ at c'., accompanied by a small amount of connective 
tissue (»<'.) from the tooth-sac. The appearance of the deep section of the 
blood-vessel is somewhat peculiar, but continuity with an undoubted capillary 
outside the organ, in this and in many other cases, leaves no doubt as to the 
true nature. It is possible that the appearance of a thick-walled or even 
solid cylinder of fusiform cells may be due to the shrinkage of a relatively 
large thin-walled vascular channel, following from the peculiarly delicate and 
fluid condition of the surrounding tissues. The absence of this latter cause 
may account for the fact that the external capillary at c. and the capillaries in 
the tooth papilla (Fig. l,c) possess an entirely normal structure and appearance. 
Fig. 5. — x 188. A portion of a section of the middle membrane of the 
enamel organ, showing the structure of its deeper part close to the stratum 
intermedium. The section was taken from the same tissue as that from 
which Fig. 1 was drawn. The figure indicates that the vascular channels [d .) 
penetrate the layer to a great depth, carrying a small amount of connective 
tissue (»/.) with them. The structure of the channel resembles that of the 
deeper part of the blood-vessel, described in the last figure (e'.). An appa- 
rent lumen is shown at l. It is probable that the vessels shown in this and 
the last figure are the main vascular channels, and that smaller branches 
form a network in the middle membrane of the enamel organ. Such a con- 
clusion was suggested by many of the sections. 
Fig. G. — X 188. From the same tissue as the last section, showing the 
structure of the epithelial nodule in the most superficial part of the middle 
membrane of the enamel organ, immediately over the apex of each chief cusp 
of the large broad posterior teeth. The relative position of cusp, nodule 
and oral epithelium is shown in many of the figures in PI. II. The nodule 
(W.) is seen to lie in the superficial part of the middle membrane of the 
enamel organ ( m . m.), and immediately below the irregular and apparently 
discontinuous outer membrane (o. m.). The tooth-sac is seen at m. A space 
(c. sp.) in the middle membrane probably contained a vascular channel. The 
nodule is seen to be made up of a dense outer tissue, composed of fusiform 
deeply-staining cells surrounding a concentric space, in which are scattered 
thin yellowish cells, with a central dense mass made up of similar cells. The 
latter are not indicated by outlines, but by the presence of minute traces of a 
nucleus. In some sections an obliquely-cut cylinder of similar structure 
appears to extend from the nodule towards the apex of the cusp, and is 
perhaps continuous with the inner layer of the enamel organ or the stratum 
intermedium in this locality. 
