18 
EDWARD B. POOLTON. 
to study the distribution of these vessels iu the higher Mam- 
malia by means of injected specimens. In Ornithorhynchus 
the vessels can be seen entering through the outer layer from 
the surrounding subepithelial tissues ; the same vessel can be 
traced from the outside into the stellate reticulum (see PI. Ill, 
fig. 4, c. and c'.). Within the latter the vascular channels are 
unlike ordinary capillaries, having the appearance of cords of 
fusiform cells with a very small lumen, which is often difficult 
to detect (figs. 4 and 5, c'.). It seems probable that this abnor- 
mality is due to shrinkage, which is in some way connected 
with the extremely soft and delicate tissue in which the chan- 
nels are embedded. Thus the change may have followed from 
the post mortem drying up or absorption of the fluid in the 
meshes of the stellate reticulum, before the animal was placed 
in spirit, or, again, it may be connected with the action of the 
spirit upon vascular walls traversing a tissue which yields them 
so little support. That the peculiarity is connected with the 
surrounding tissues seems clear from the normal character of 
the capillaries in the tooth papilla (PI. Ill, fig. 1, c.), and 
in the subepithelial tissues (fig. 4, c .) ; and yet continuity 
between the channels in the stellate reticulum and the ex- 
ternal normal capillaries (fig. 4) clearly indicates the vascular 
nature of the former. Many appearances seemed to show 
that channels such as have been represented in the figures — 
although very numerous — are only the main vessels of the 
layer, and that a much finer network of smaller vessels is also 
present. A thin layer of subepithelial (mucosa and submucosa) 
connective tissue appears to be invaginated with the former 
system of vessels (PI. Ill, fig. 4, to'., which is seen to be 
continuous with m. ; also see fig. 5, m'.) Such vessels often 
penetrate in a radiate manner to a great depth, nearly reaching 
the stratum intermedium. Thus fig. 5 represents such a deep- 
seated position. It seems probable that the chief vascular inva- 
ginations represent a further specialization of the papilliform 
processes which are well known to invaginate the outer epi- 
thelium into the stellate reticulum of the enamel organ (Tomes, 
1. c., p. 134). 
